Reword invitation text on front page

The new text is friendlier and less presumptuous and actually describes
the reason why I introduced the invitation system. Not sure why I
didn't write that in the first place.

Thanks to <https://lobste.rs/about> for the idea.

Signed-off-by: Florian Pritz <bluewind@xinu.at>
Fix comptability with php 5.6

References: http://ellislab.com/forums/viewthread/244510/#1066558

Signed-off-by: Florian Pritz <bluewind@xinu.at>
update gentoo overlay URL

Signed-off-by: Florian Pritz <bluewind@xinu.at>
Move migration code to CLI callable function

This is necessary to prevent migrations from running multiple times in
parallel. A git hook can be used to run this after checkout so impact
should be fairly low.

Signed-off-by: Florian Pritz <bluewind@xinu.at>