Various fixes after group testing

[cosmicespresso]

Closes getlantern/engineering#946 getlantern/engineering#948 getlantern/engineering#691 getlantern/engineering#685 getlantern/engineering#688 and fixes strings in en.po here and there.

SVID_20210914_113529_1.mp4

• All strings are defined using localized message keys like 'my_message_key'.i18n and are defined in en.po.
• All colors are defined using Hex (not using built-in colors), are defined in colors.dart and are not duplicated
• All text styles are defined in text_styles.dart and are not duplicated
• All icons are using the vector resource from Figma (do not use built-in Icons)
• Repeated code has been factored into custom widgets
• Layout looks good in both LTR (English) and RTL (Persian) languages
• If you refactored existing code, have you tested the refactored functionality against the old version to make sure you didn't break anything?
• Do the tests pass? Consistently?
• Did this change improve test coverage?
• Is the code in question being linted? If not, consider adding a linter step to CI. If yes, make sure the linter is happy.
• Have you logged tickets for related technical debt with the label “techdebt”?

[cosmicespresso]

Back button navigation is working perfectly for me! It may not be related to this PR, but I noticed that the emoji keyboard seems smaller than before(?).

@ox I experimented/tested on my phones and updated things a little in 2f2b7e0. Still, pretty hacky obviously. Looks like triggering the native emoji keyboard is hard, as is getting a measurement of the native keyboard height from Flutter (so that we can size them equally). Let me know how it looks on your phone? What I'm more worried about is just how laggy using it is, does not make up for smooth UX unfortunately. Maybe we can ticket something in the backlog for iterating on it?

SVID_20210915_142426_1.mp4

[oxtoacart]

What I'm more worried about is just how laggy using it is

In release builds the Emoji keyboard doesn't seem too laggy to me. Is yours slow even on the release build from Slack @Kallirroi ?

[oxtoacart]

Nice work @Kallirroi !