MDL-43312 TinyMCE: Version bump to 3.5.10. Remove all the hacks.

I retested the old bugs that required custom hacks to tinymce and could not reproduce them,
so I have not reimplemented the hacks.
MDL-43312 TinyMCE: Reapply the z-index hacks (25 only)
MDL-43312 TinyMCE: Add back the upgrade instructions for z-index on 2.5
Merge branch 'MDL-43312-25' of git://github.com/damyon/moodle into MOODLE_25_STABLE
MDL-45414 wiki: blank image for page index icons
MDL-45332 repository_url: URL parameter is using PARAM_URL
MDL-44262 Behat: Bump behat version to get format fix
Merge branch 'wip-mdl-44262-m25' of https://github.com/rajeshtaneja/moodle into MOODLE_25_STABLE