Revert "bumping to 0.5.2 for dev purposes"

This reverts commit e6824fd727abaf54e49d9616d20c814a9b06a88e.
0.5.2-dev in ACP, just so people know it's not a released version
check length
typo
fixing upgrade text to use nodebb executable, removed exposure of option to run without upgrading
closes #2209
closes #2175
fixed issue where non-admins would get maintenance mode header if maintenance mode was NOT enabled (logic errors, grrr..)
added jquery drag and drop event libs
created selectable module for selecting / grouping elements
moved module less files out of components and into a separate folder
Merge branch 'master' of https://github.com/NodeBB/NodeBB
closed #2224
closes #2223, ability to select multiple tags
fixed tag searching
fixed tag selectability after search
fixed selectable
closes #2221
selectable - click/shiftclick to select as well
fixed #2220