Backport 0.6 fixes and non-compat-breaking features.

[KitsuneRal]

• Room::aliases() and Room::setAliases() never work #301, using the code from fixing msc2432 #383 (but without new API for RoomCanonicalAlias)
• #anything:... is not a valid alias #321
• Server notices room tag #326 (using IncorrectRequest instead of a newly introduced error code)
• .well-known support #330
• Reactions and edited messages support #341 (if possible)
  • MSC1849 compatible edited messages #373
  • Simple changes for sending edits #376
• Support M_USER_DEACTIVATED #344 (similarly, using ContentAccessError to preserve the API)
• Update the cache for sure if unread counters were changed #345
• Crash: QMatrixClient::User::Private::avatarUrlForRoom(QMatrixClient::Room const*, QUrl const&) const + 51 #347
• All jobs send access token #358
• Room highlighting misses complex room names. #359
• No read receipt in freshly made rooms #361
• MSC1954: Remove prev_content from the essential keys list #365
• Trouble downloading files with colons in the name #366
• Error compiling: redefinition of ‘struct std::hash<QString>’ #387
• SSO backend #388
  • Support detecting SSO #386
• Alias updates don't trigger namesChanged() #390
  Bonus:
• Expose the list of known-left rooms #354