Bug 40925: Move the security level backend to tor-browser.git

First stage of the security level reviews. Fixes #40925 partly.

This MR:

• moves the backend from torbutton to tor-browser (I have a related commit for torbutton, but we do not have a maint-11.5 branch, yet)
• remove dependency on strings from TorStrings.jsm, and migrate to fluent for strings

Problems/TODO:

• the code could be still improved (especially, the backend, it's almost a copy&paste)
• we may want to rename the preferences, and to implement a mechanism to migrate from the old ones to the new ones
• English will be available, until we fix also #40926 (closed)

The first two points depend on the decision on how we want to implement the security level (i.e., keep a dependency on NoScript, or add some code to tor-browser).