Fix Postgres CPU burn

According to Heroku, we have a query that accounts for 80% of all time spent in postgres

```
$ heroku pg:outliers
 total_exec_time  | prop_exec_time |   ncalls    |   sync_io_time   |                                                                                       query
------------------+----------------+-------------+------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 3790:50:52.62102 | 80.2%          | 100,727,265 | 727:08:40.969477 | SELECT  ? AS one FROM "repos" WHERE LOWER("repos"."name") = LOWER($1) AND ("repos"."id" != $2) AND "repos"."user_name" = $3 LIMIT $4
```

Where did it come from? Turns out, it's not in my code. It was added by a rails validation.

But, you might be thinking. We only have 2,000 repos in the whole database, why is it updating so much?

Well, it turns out this validation is called on EVERY update.

For example this sidekiq task:

```
Jun 29 07:00:32 issuetriage app/scheduler.8183:  [ActiveJob] Enqueued PopulateIssuesJob (Job ID: 9e04e63f-a515-4dcd-947f-0f777e56dd1b) to Sidekiq(default) with arguments: #<GlobalID:0x00000004f98a68 @uri=#<URI::GID gid://code-triage/Repo/1008>>
Jun 29 07:11:05 issuetriage app/worker.1:  [ActiveJob] [PopulateIssuesJob] [9e04e63f-a515-4dcd-947f-0f777e56dd1b] Performing PopulateIssuesJob (Job ID: 9e04e63f-a515-4dcd-947f-0f777e56dd1b) from Sidekiq(default) with arguments: #<GlobalID:0x007fd098f6bdf8 @uri=#<URI::GID gid://code-triage/Repo/1008>>
Jun 29 07:11:05 issuetriage app/worker.1:  [ActiveJob] [PopulateIssuesJob] [9e04e63f-a515-4dcd-947f-0f777e56dd1b]   User Load (10.4ms)  SELECT  "users".* FROM "users" WHERE ("users"."github_access_token" IS NOT NULL) ORDER BY RANDOM() LIMIT $1
Jun 29 07:11:05 issuetriage app/worker.1:  [ActiveJob] [PopulateIssuesJob] [9e04e63f-a515-4dcd-947f-0f777e56dd1b]    (35.4ms)  BEGIN
Jun 29 07:11:05 issuetriage app/worker.1:  [ActiveJob] [PopulateIssuesJob] [9e04e63f-a515-4dcd-947f-0f777e56dd1b]   Repo Exists (352.9ms)  SELECT  1 AS one FROM "repos" WHERE LOWER("repos"."name") = LOWER($1) AND ("repos"."id" != $2) AND "repos"."user_name" = $3 LIMIT $4
Jun 29 07:11:05 issuetriage app/worker.1:  [ActiveJob] [PopulateIssuesJob] [9e04e63f-a515-4dcd-947f-0f777e56dd1b]   SQL (3.7ms)  UPDATE "repos" SET "github_error_msg" = $1, "updated_at" = $2 WHERE "repos"."id" = $3
Jun 29 07:11:05 issuetriage app/worker.1:  [ActiveJob] [PopulateIssuesJob] [9e04e63f-a515-4dcd-947f-0f777e56dd1b]    (4.5ms)  COMMIT
Jun 29 07:11:05 issuetriage app/worker.1:  [ActiveJob] [PopulateIssuesJob] [9e04e63f-a515-4dcd-947f-0f777e56dd1b] Performed PopulateIssuesJob (Job ID: 9e04e63f-a515-4dcd-947f-0f777e56dd1b) from Sidekiq(default) in 629.22ms
```

Going to remove the validation and do it in the DB instead, which is better anyway. However it doesn't give us the nice errors of a rails validation (I don't think). Would be nice to integrate this in the future.

Ideally `validates` would see if there is a unique index and use that instead, give us a warning if no unique index existed, because race conditions.

app/models/repo.rb

@@ -2,7 +2,8 @@
 
 class Repo < ActiveRecord::Base
   validate :github_url_exists, on: :create
-  validates :name, uniqueness: {scope: :user_name, case_sensitive: false }
+
+  # Now done at the DB level # validates :name, uniqueness: {scope: :user_name, case_sensitive: false }
   validates :name, :user_name, presence: true
 
   has_many :issues

db/migrate/20170629151616_add_unique_index_to_repos.rb

@@ -0,0 +1,5 @@
+class AddUniqueIndexToRepos < ActiveRecord::Migration[5.1]
+  def change
+    add_index :repos, [:name, :user_name], :unique => true
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170627024417) do
+ActiveRecord::Schema.define(version: 20170629151616) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -153,6 +153,7 @@
     t.integer "stars_count", default: 0
     t.index ["issues_count"], name: "index_repos_on_issues_count"
     t.index ["language"], name: "index_repos_on_language"
+    t.index ["name", "user_name"], name: "index_repos_on_name_and_user_name", unique: true
     t.index ["name"], name: "index_repos_on_name"
     t.index ["user_name"], name: "index_repos_on_user_name"
   end