[feature] Admin web page for instance management

[tsmethurst]

We should have a web page served at https://example.org/admin (and sub-paths) which allows an instance admin to manage settings for the instance. Web pages should follow the templating pattern established for the instance base path by @f0x52, for showing current settings, and make simple POST or PATCH form requests for updating information.

An admin should be able to:

• view and update instance information
• create, view, delete domain blocks
• import and export domain block lists

Instance information updating eg https://example.org/admin/instance

Current instance info can be retrieved by calling /api/v1/instance (see eg: https://testingtesting123.xyz/api/v1/instance), which calls the Processor function here: https://github.com/superseriousbusiness/gotosocial/blob/main/internal/processing/instance.go#L31

Instance info can be updated by calling a PATCH to /api/v1/instance, which calls the Processor function here: https://github.com/superseriousbusiness/gotosocial/blob/main/internal/processing/instance.go#L45

The form to submit for an update is modeled here: https://github.com/superseriousbusiness/gotosocial/blob/main/internal/api/model/instance.go#L77

Domain block creation, viewing, and deletion eg https://example.org/admin/federation

Admin should be able to view a list of current domain blocks, probably with pagination since there could be a lot of them (pixie.town and rage.love for example both have hundreds of domain blocks).

Domain blocks can be retrieved by calling /api/v1/admin/domain_blocks, which calls the Processor function here: https://github.com/superseriousbusiness/gotosocial/blob/main/internal/processing/admin.go#L39

A new domain block can be created by a POST to /api/v1/admin/domain_blocks, which calls the Processor function here: https://github.com/superseriousbusiness/gotosocial/blob/main/internal/processing/admin.go#L31 with the following model: https://github.com/superseriousbusiness/gotosocial/blob/main/internal/api/model/domainblock.go#L36

An individual domain block can be viewed by calling /api/v1/admin/domain_blocks/DOMAIN_BLOCK_ID, and deleted by a DELETE to that path.

Domain blocklist import/export https://example.org/admin/federation

To mass import domain blocks, a form can be submitted as a POST to the /api/v1/admin/domain_blocks path given above, where the form field domains is a json file containing an array of domains to block, and the query parameter import is set to true. See here: #77

An admin should be able to upload this file through the admin page as well.

To retrieve a list of blocked domains for sharing elsewhere, without all the domain block IDs and other extraneous information, an admin can make a GET to /api/v1/admin/domain_blocks with the query parameter export set to true. -- This should also be possible through the admin page, a la Grafana's 'export this' functionality where it returns some copy-pasteable JSON.

[f0x52]

Basic panel now lives at https://github.com/superseriousbusiness/gotosocial-admin, checks off all the features except there's currently no way to see the details of a domain block