Moved default zoom level from Advanced to General prefs

[luixxiul]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Closes #5697

Auditors: @bsclifton @bradleyrichter

Test Plan:

1. Open about:preferences#general
2. Make sure the default zoom level exists between Language and Import Browser Data
3. Open about:preferences#advanced
4. Make sure the default zoom level no longer exists

[bsclifton]

Looks great! thanks 😄