Patch the 'm.room.power_levels' event if present instead of completely overwriting it

[digitalentity]

Addresses some of the #5:

• Adds a configuration setting patch_power_levels (default disabled)
• Fixes the docs mentioning the option, how it works and why it may be dangerous
• Patch the event instead of owerwriting it if already present

[digitalentity]

I was also thinking about the exact same "find the max power level and bump it by 50" logic!

Yeah, I've figured it would be both more reliable and simpler to implement.

[digitalentity]

Tested on Synapse with FluffyChat (Android+Web) and Element (Web and Linux). Merging.