Use shared reader css file

[malinajirka]

Fixes #11673
Fixes #7636
Fixes #11685
Fixes #11779 (I believe it's desired behavior)

The main goal of this PR is to use mobile specific (shared) css file for rendering posts in the Reader. The css file is heavily based on Calypso's Reader css file. This PR also changes some of the hardcoded styles to fix some minor issues.

Questions:

1. Should/Can we move the hardcoded styles into the shared mobile css file?
2. As we discussed this implementation uses an infinite cache - it never gets invalidated. Do we want to introduce some automatic cache invalidation or we are okay with an infinite cache which can be invalidated only by manually changing the code?

To test:

• Test any posts/blocks you can think of.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[leandroalonso]

As we discussed this implementation uses an infinite cache - it never gets invalidated. Do we want to introduce some automatic cache invalidation or we are okay with an infinite cache that can be invalidated only by manually changing the code?

For iOS I implemented a class to manage that:

https://github.com/wordpress-mobile/WordPress-iOS/pull/14357/files#diff-33e1d7c630ae0273ff430172c91cc535

I know this is not optimal and ideally, the server should handle that but is enough for now. Although I'm really not sure about the amount of time.

[malinajirka]

Thanks for the review!!!

For iOS I implemented a class to manage that:
https://github.com/wordpress-mobile/WordPress-iOS/pull/14357/files#diff-33e1d7c630ae0273ff430172c91cc535

Thanks for the info, I'll implement the same class on Android;). Definitely way better than manual updating:).