Download page: make stable release link stand out more
Download page: reworded text blob for dev release
Download page: remove hosted link from stable release box

This is redundant, we already have the same link just below the title.
Download page: remove MantisTouch link from stable release box

This is redundant, we already have this in the Add-On software box
further down the page.
Download page: link to roadmap instead of changelog for dev release
Requirements page: global update, fix broken links
Download page: add link to Github
Download page: reword text, add Travis CI link and status icon
Download page: CSS and layout adjustments
Download page: reword and rearrange text for distributions
Download page: move upgrade instructions to Stable release box

Also added a link to the relevant Admin Guide page
irclogs page: revise CSS to make table styles generic
irclogs page: added subtitle
irclogs page: fix error handling when missing dir
Add new Nightly Builds page

This replaces the default apache directory listing we had in place until
now to list the nightly builds downloads.

The page includes Travis CI build status icons
Contact us page: fix deprecated use of assignment by reference
Builds page: SplFileInfo::getExtension not available

mantisbt.org is on PHP 5.3.2 and SplFileInfo::getExtension() was only
introduced in 5.3.6 so we use a workaround.
Builds page: ignore dirs when scanning /builds
Builds page: fix the paths on file links
New Nightly Builds and improved Downloads pages

Fixes #16212, #16213