Convert more activities to derive from WebViewerActivity.

To do that, move some code that is not specific to the web viewer to the
respective activities.
Use WebViewerActivity to fix a number of problems:
- Fix the search field foreground color for DarkActionBar theme
- Properly handle dark theme for wiki and blog activities
- Show spinning circle until page is fully loaded
Deduplicate code.
Deduplicate more code and remove dead code.
Switch to new Github bindings.

Those initialize the comment position to -1 if it isn't included in
JSON.
Don't show line comments in commit comment list.
Cleanup.
Fix editing pull request comments.
Remove duplicate divider on top of comment list.
Indicate number of comments in commit file list.

In order to not duplicate stuff, clean up the comment icon code by
creating an attribute resource for it and assigning the drawable from
XML.
Deduplicate more code.
Avoid fetching comments more often than needed.
Properly filter outdated comments.
Fix multi-line comments taking large amounts of space.
Properly handle renames.

Renames with no additional changes have no patch attached, so we
shouldn't make those clickable. As a non-clickable entry under 'Files
Changed' would be a little confusing, introduce a separate header for
renamed files.
Restore possibility to view changed image files.
Refactor issue list activity.

In order to make it consistent to the pull request activity, have
open/closed tabs instead of having 3 tabs that present the same data in
different sort orders. Make the sort order selectable by a menu instead.
Fix unsetting assignee and milestone.

The bindings expect user and milestone to be set; if they're null it
means 'don't touch it'.
Use the correct selected background on API 19.
Fix commit file item padding and use a consistent style for header
labels.
Overhaul issue and milestone activities.

Provide a little more padding (and make the padding consistent in
general) and fix code duplication between milestone creation and
editing.