Add constants for mention room power levels.

[pixlwave]

Adds kMXRoomPowerLevelNotificationsRoomKey and kMXRoomPowerLevelNotificationsRoomDefault and allows encrypted messages to be highlighted if the contain @room.

Part of element-hq/element-ios#7453

[codecov]

Codecov Report

Patch coverage: 21.05% and no project coverage change.

Comparison is base (630d112) 17.81% compared to head (7fb733d) 17.82%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1750   +/-   ##
========================================
  Coverage    17.81%   17.82%           
========================================
  Files          615      615           
  Lines        97009    97026   +17     
  Branches     40685    40696   +11     
========================================
+ Hits         17285    17291    +6     
- Misses       79213    79223   +10     
- Partials       511      512    +1     

Impacted Files	Coverage Δ
MatrixSDK/JSONModels/MXRoomPowerLevels.m	0.00% <ø> (ø)
...ter/Checker/MXPushRuleEventMatchConditionChecker.m	64.70% <21.05%> (-26.48%)	⬇️

... and 1 file with indirect coverage changes

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[stefanceriu]

lgtm 👍