Update CODEOWNERS to add templates to frontend list and shared code within mathesar directory to maintainers list

[pavish]

This PR:

• Adds /mathesar/templates/ to frontend list
• Adds /mathesar/views.py and /mathesar/urls.py to maintainers list

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the master branch of the repository
• My commit messages follow best practices.
• 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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Developer Certificate of Origin

Developer Certificate of Origin
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(a) The contribution was created in whole or in part by me and I
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(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
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[codecov-commenter]

Codecov Report

Merging #1000 (701f3f6) into master (809b494) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1000   +/-   ##
=======================================
  Coverage   93.29%   93.29%           
=======================================
  Files          86       86           
  Lines        3161     3161           
=======================================
  Hits         2949     2949           
  Misses        212      212           

Flag	Coverage Δ
pytest-backend	93.29% <ø> (ø)

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