add trap focus to modals

[ayanchoudhary]

Fixes

Fixes #1052 by @zackkrida

Description

Introduces trap focus in AppModal and SafeBrowsingModal

Technical details

Uses focus-trap-vue

Tests

Screenshots

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
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• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[zackkrida]

Awesome!