[DS-2052] @mire Mirage 2 Contribution

[DS-2052] @mire Mirage 2 Contribution
fixed slow build
build with dependencies included now default
Update general.xsl

missed some debugging code
added support for multiple mirage 2 versions
added support for multiple mirage 2 versions - for deps included mode
cleanup after multiple mirage 2 versions addition
fixed an issue where the thumbnail placeholder wouldn't be shown in discovery results
name change
Added sections on customisation and using multiple themes
Fixed a typo
move mirage2 messages and dynamically fill theme name in sitemap
Merge branch 'mirage2-contribution' of https://github.com/atmire/DSpace into mirage2-contribution
Added missing filtering for the dependencies included install.
Removed accidentally committed mirage2 as default xmlui.xconf
Removed references to removed Mirage2 aspect.
Merge pull request #587 from atmire/mirage2-contribution

[DS-2052] @mire Mirage 2 Contribution