Downgrade to GitHub Java API 2.1.2

This is only temporary until 2.1.4 syncs to Maven Central
and then that version will be used.
Markdown files render in commit view

Now when you click on the header for a markdown file in the commit
list view it acts the same way as if you were viewing the file in
the source tree view (i.e. the file can either be rendered or shown in
its raw form)

Closes #343
DRY up markdown extension checking

Create new utils package class to check if file name
has a markdown extension.
Downcase file name before comparing against Markdown extensions
Upgrade to GitHub Java API 2.1.4
Update Turkish translation
Remove unneeded casts and use String.contains()
Remove unneeded instanceof check

Repository implements Serializable and does not need
to be explicitly checked before inserting into the bundle.
Always call setCurrentItem from updateCurrentItem

Previously updateCurrentItem() was only called if pager.setItem()
returned true which would not be the case when the pager was manually
swiped since the position had already been set to the new value.

Closes #352
Remove full HTML when raw HTML value changes

Previously the issue body html was not being re-encoded
after edits and also the full html was never being flushed
when the raw version has changed from the last time it was
encoded.

This would cause the issue description to not update after
being edited.

Closes #353