Github: add spacer rows around Application setup
Github: rewording application help text strings

Make it clearer what the Github Application Client ID and Secret are
used for.
Fix PHP notice
Use specific error message for empty required field
Fix crash caused by infinite loop

When there is no data in the changeset table, Source_Date_Select()
goes into an infinite loop and crashes as PHP runs out of memory.
Github: fix broken link for Browse without SHA ref
Add 'Search' link to changesets list page
Add 'Back to Index' link to search results page
Do not interpret HTML tags in changeset messages

When displaying changesets, the commit message is now manually
transformed (adding href, bug and bugnote links + nl2br) instead of
calling string_display_links(), which avoids unwanted html tags
processing by the MantisCoreFormatting plugin.  Rationale: commit
messages being plain text, any html they may contain should not be
considered as formatting and must be displayed as-is.

Fixes #93