Fix URL base for releases from GitHub RSS feed

Links from releases feed are specified without protocol, hostname
and port, so we must append https://github.com to them.

Reported-by: Tomasz Żok <tomasz.zok@gmail.com>
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Fix URL for released OfflineIMAP versions

Reported-by: Tomasz Żok <tomasz.zok@gmail.com>
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Fix wording and URL for the last stable release

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>