[feature] Implement /api/v1/reports endpoints on client API

[tsmethurst]

Description

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This pull request implements the following client API endpoints, as part of our upcoming report/flag functionality:

GET /api/v1/reports -- view all reports submitted by authenticated user. Parameters are available to filter which reports are returned, based on the query parameters here: https://docs.joinmastodon.org/methods/admin/reports/
GET /api/v1/reports/{id} -- view report by authenticated user with given ID
POST /api/v1/reports -- create a new user report

Some design comments:

• The mastodon api supports setting category when submitting reports. I don't think this is a concept we really need, so I didn't include it. Reports will always have the default 'other' category.
• The mastodon api also support submitting an integer array of rules that have been violated, when submitting reports. Since we don't have rules yet, I didn't implement this. We can add it in the future if we implement lists of rules.

While these endpoints create reports in the database, the side effects (emails, federation) are not yet handled. Will do this in an upcoming PR.

relates to #1309

Checklist

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• 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.