shanbady/separate database router and schema for program certificates

[shanbady]

What are the relevant tickets?

Closes #3771

Description (What does it do?)

The changes in this PR properly migrate the ProgramCertificate table to its own schema and also includes a database router that prevents writes from happening on tables in the external schema.

How can this be tested?

1. checkout this branch
2. run the migrations or do a full docker-compose down+up
3. in the postgres database see that there is an external schema. in manage.py dbshell you can run the following:

 SELECT nspname
 FROM pg_catalog.pg_namespace;

4. trying to create a programcertificate in django shell manually should give you an exception:

from profiles.models import ProgramCertificate
ProgramCertificate.objects.get_or_create(user_full_name='john doe',user_email='test@test.com', micromasters_program_id=1, program_title='Supply Chain Management')

Additional Context

Our previous deploy of the programcertificate table ended up create a table in the public schema named "external.programcertificate".

The new migrations in this PR do the following to fix that:

1. Drop the rogue programcertificate table
2. recreate the programcertificate table in the external schema

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.75%. Comparing base (f125f24) to head (07d2eb6).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #617      +/-   ##
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+ Coverage   74.73%   74.75%   +0.02%     
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  Files         245      246       +1     
  Lines       11170    11180      +10     
  Branches     1943     1944       +1     
==========================================
+ Hits         8348     8358      +10     
  Misses       2648     2648              
  Partials      174      174              

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[ChristopherChudzicki]

👍

[ChristopherChudzicki]

fun.