move FLAG_SECURE to PassphraseRequiredMixin

Fixes #1402
// FREEBIE
Merge pull request #1614 from mcginty/contact-list-security

move FLAG_SECURE to PassphraseRequiredMixin
add section about submitting useful bug reports
remove ReviewIdentitiesActivity

// FREEBIE
temporarily disable encrypted backup

// FREEBIE
Merge pull request #1631 from mcginty/disable-encrypted-export

temporarily disable encrypted backup
Fixing race condition and other mistakes. Fixes #1603.

// FREEBIE
Merge pull request #1630 from mcginty/remove-keys-list

remove ReviewIdentitiesActivity
Remove unused files.

Fixes #1522

// FREEBIE
Replace XML serializer in plaintext export

Fixes #342

- using regex pattern/matcher to escape chars below 0x0020 and
  above 0xd7ff
- using String.Replace to escape XML entities
- changed XmlPullParser from Xml.newPullParser() to
  XmlPullParserFactory parser to fix import on GB
Revert "change out key cached icon to be more unique"

This reverts commit d6d76fa95315068ab5457b23fb4908c4a8586982.
no longer load a recipient for each filter text

// FREEBIE
Merge pull request #1642 from mcginty/canonical-bunk-addresses

don't try to load recipients for each filter text
Add xxhdpi check drawable for notifications

Fixes blurry mark as read icon on Android Wear. FREEBIE.
Merge pull request #1678 from veeti/check-icon

Add xxhdpi check drawable for notifications
use latest android number as recipient number

Fixes #791
// FREEBIE
Update language translations.

// FREEBIE
Bumping version to 2.1.0
Upgrade to latest Android gradle plugin

// FREEBIE
Closes #1660
Add date to saved media filenames

Fixes #1689
Closes #1693