[frogend] Filter local emoji

[f0x52]

Description

Small quality-of-life improvement to the local emoji list, adding a bit of explanation and a text field to filter the list (similar to the federation domain list).

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[f0x52]

(Needs to be rebased after #1362 is merged, since it builds upon those CSS changes)

[tsmethurst]

lgtm! :)