Empty contentpack for distribution and testing

[benjaoming]

Summary

We should finally kill off the English content pack, as the only purpose it serves is to have a function content tree in a blank installation -- for which users should download a full English content pack, anyways.

This will also greatly improve test speeds AFAIK.

TODO

If not all TODOs are marked, this PR is considered WIP (work in progress)

• [x] Will need to deal with whatever assumptions our tests have been based on by writing a bit of fixtures on top of the empty content db.
• [x] Make sure the Language tab communicates that it's an empty English language pack.
• [x] Make sure the Language tab tells users downloading other languages that they MUST install the English language pack.
• [x] Have tests been written for the new code? If you're fixing a bug, write a regression test (or have a really good reason for not writing one... and I mean really good!)
• [x] Has documentation been written/updated?
• [x] New dependencies (if any) added to requirements file
• [x] ~After factoring out illegal <a> elements, make sure cursor:pointer is restored~ Restored because of accessibility concerns, @radinamatic look, you didn't even have to point it out :)
• [x] Ensure that the test content.db does not get rebuild before every BDD test, just build it once.

Reviewer guidance

Please comment if you have any concerns about testing with no content data :) IMO we need to speed up the tests significantly, so going from 92MB test db to ~0 MB is pretty awesome.

Issues addressed

5336 #5318

[codecov-io]

Current coverage is 51.81% (diff: 78.63%)

Merging #5350 into develop will increase coverage by 0.79%

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  Methods           0          0          
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+ Hits           3820       3942   +122   
+ Misses         3667       3666     -1   
  Partials          0          0          

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[benjaoming]

All BDD tests are actually working now, other tests are failing because they haven't got a content.db with fixtures.

[benjaoming]

Update on English content pack vs. empty content.db:

If there is an empty content.db (i.e. no content packs installed), the user will now be displayed this warning regardless of the situation. For instance, if you have 1 other language, it will still show up. I think this is the intention of all content packs as they do not contain any static exercise data files.