Support multiple RCs in the Beta Stability Pushlog page

[marco-c]

Currently, only 49.0b99 is supported because of https://bugzilla.mozilla.org/show_bug.cgi?id=1301782 and https://bugzilla.mozilla.org/show_bug.cgi?id=1192197 (#16).

[marco-c]

A hacky solution would be to guess the tag and the release date using the build ID of the RC.
I'd like to avoid it, so I'll wait a while hoping that those bugs will be fixed.

[marco-c]

We can take the info from URLs like this: http://ftp.mozilla.org/pub/firefox/candidates/49.0-candidates/build1/win32/en-US/firefox-49.0.txt.