Disable spellcheck for custom filters list

[cndouglas]

• 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).

Test Plan:

1. Navigate to about:adblock.
2. Type random text in the custom filters text area.
3. Make sure the text does not have a red underline (spellcheck).

Fixes #4619

[bbondy]

thanks!