feat: added composer category selector

[ethan1696]

This PR changes the category selector in the composer when posting a topic. It changes it to use checkboxes so a post can be made to multiple categories at once.

This addresses #22 and #23

[coveralls]

Pull Request Test Coverage Report for Build 13552409986

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.005%) to 82.676%

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Files with Coverage Reduction	New Missed Lines	%
src/meta/errors.js	1	76.74%

Totals
Change from base Build 13552322057:	-0.005%
Covered Lines:	22353
Relevant Lines:	25615

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💛 - Coveralls

[kzhang31415]

Looks good, but do you think the plugin can be added as a dependency rather than directly to the code base?

[ethan1696]

Looks good, but do you think the plugin can be added as a dependency rather than directly to the code base?

Yes, the plugin is already a dependency.