Concurrency level settings use technical terminology and have default value issues

[Steve Dodier-Lazaro]

The "concurrency level" settings are a bit too technical for our average user. Also, they probably don't need to be on the same level as actual "participant control" settings.

Could we please:
- introduce a "Advanced settings" or "Performance settings" page
- modify the tooltip to: "When you log in to a website, we calculate the hash of the password you type and of similar passwords (to help us guess when you use similar passwords on multiple sites). This setting determines how fast the extension will compute the hashes. If your computer is significantly slower than usual when you log in, decrease this number. The default value is 6."
- currently, the default value in the tooltip is 6, but it's 5 in the combobox that changes the setting. Please use a *single* constant throughout the codebase (and adjust with -1/+1 for combobox indexes if necessary)