Request reactions data from GitHub (#839)

This commit updates the GitHub graphql query to also fetch reactions.
We update the JSON typedefs to include this new information, add
continuations from comments, and update existing continuation and query
code. Also, I added a safety check when updating comments for issues
that was previously unnecessary but is now needed.

Test plan:
- `yarn test --full` passes.
- Setting the page limits to 1 and running on the example-github does
not error with unexhausted pages, and loads all the expected reactions.
- Running on a larger repository (go-ipfs) works as expected.
- I have written dependent code that consumes these reactions in the
RelationalView, and works as intended, which suggests that the type
signatures are correct.

src/plugins/github/__snapshots__/graphql.test.js.snap

@@ -259,6 +259,9 @@ fragment issues on IssueConnection {
     comments(first: 20) {
       ...comments
     }
+    reactions(first: 5) {
+      ...reactions
+    }
   }
 }
 fragment pulls on PullRequestConnection {
@@ -283,9 +286,12 @@ fragment pulls on PullRequestConnection {
     comments(first: 20) {
       ...comments
     }
-    reviews(first: 10) {
+    reviews(first: 5) {
       ...reviews
     }
+    reactions(first: 5) {
+      ...reactions
+    }
   }
 }
 fragment comments on IssueCommentConnection {
@@ -300,6 +306,9 @@ fragment comments on IssueCommentConnection {
       ...whoami
     }
     body
+    reactions(first: 5) {
+      ...reactions
+    }
   }
 }
 fragment reviews on PullRequestReviewConnection {
@@ -332,6 +341,9 @@ fragment reviewComments on PullRequestReviewCommentConnection {
     author {
       ...whoami
     }
+    reactions(first: 5) {
+      ...reactions
+    }
   }
 }
 fragment commitHistory on CommitHistoryConnection {
@@ -353,5 +365,18 @@ fragment commit on Commit {
       ...whoami
     }
   }
+}
+fragment reactions on ReactionConnection {
+  pageInfo {
+    hasNextPage
+    endCursor
+  }
+  nodes {
+    id
+    content
+    user {
+      ...whoami
+    }
+  }
 }"
 `;