Issue #3110064 by idebr, chandrashekhar_srijan, Vidushi Mehta, ckng, benjifisher, mikelutz: Migrate empty, <nolink> and <none> link field

Issue #2924860 by Kng, AdamPS: “Uncaught exception” page contains HTML but has content-type text/plain

Issue #2301317 by pwolanin, dawehner, Wim Leers, effulgentsia, YesCT, xjm, alexpott: MenuLinkNG part4: Conversion.

Revert "Issue #2301317 by pwolanin, effulgentsia, Wim Leers, dawehner, alexpott: MenuLinkNG part4: Conversion."

This reverts commit fd2db9cd.

Issue #2301317 by pwolanin, effulgentsia, Wim Leers, dawehner, alexpott: MenuLinkNG part4: Conversion.

Issue #2301313 by pwolanin, dawehner, Wim Leers, effulgentsia, kgoel, YesCT, glide: MenuLinkNG part3 (no conversions): MenuLinkContent UI.

Issue #2301273 by pwolanin, dawehner, Wim Leers, effulgentsia, joelpittet, larowlan, YesCT, kgoel: MenuLinkNG part2 (no UI or conversions): MenuLinkTree API and unit tests.

Issue #2301239 by pwolanin, dawehner, Wim Leers, effulgentsia, joelpittet, larowlan, xjm, YesCT, kgoel, victoru, berdir, likin, and plach: MenuLinkNG part1 (no UI or conversions): plugins (static + MenuLinkContent) + MenuLinkManager + MenuTreeStorage.