[feature] Configurable max profile fields (#4175)

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

Profile fields, right? So I made them a configurable amount and show them on the instance features. Closes #1876

All changes I did are described in the commits. I tried to cover as much as possible, but I don't know this codebase to know what I've missed. I have tested it manually and it works (settings page, `/api/v1/instances`, server-side validation).

Not done:
- An "Add profile fields" button, so that instances with a high profile fields count (e.g. 100) aren't an issue when trying to skip the profile fields (in some form, accessibility)
- Updating the swagger docs that specify a specific amount of profile fields for the updating endpoint (not sure how to proceed with that)
- Unchecked checklist items

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [X] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [X] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [X] 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.
- [X] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [X] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Notes about checklist:
- ["Approval" to work on this reference](https://matrix.to/#/!mlPctfMHjyopbOnliM:superseriousbusiness.org/$P0Z1Qmny6GNIgxhre69gll8KSD690HC8nVvNpXmu3nU?via=superseriousbusiness.org&via=matrix.org&via=tchncs.de)
- `golangci-lint run` output ignored due to complaining about ffmpeg and some other code I didn't touch

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4175
Reviewed-by: kim <gruf@noreply.codeberg.org>
Co-authored-by: Jackson <jackson@jacksonchen666.com>
Co-committed-by: Jackson <jackson@jacksonchen666.com>

test/envparsing.sh

@@ -7,6 +7,7 @@ EXPECT=$(cat << "EOF"
     "account-domain": "peepee",
     "accounts-allow-custom-css": true,
     "accounts-custom-css-length": 5000,
+    "accounts-max-profile-fields": 8,
     "accounts-reason-required": false,
     "accounts-registration-backlog-limit": 100,
     "accounts-registration-daily-limit": 50,
@@ -247,6 +248,7 @@ GTS_INSTANCE_LANGUAGES="nl,en-gb" \
 GTS_INSTANCE_STATS_MODE="baffle" \
 GTS_ACCOUNTS_ALLOW_CUSTOM_CSS=true \
 GTS_ACCOUNTS_CUSTOM_CSS_LENGTH=5000 \
+GTS_ACCOUNTS_MAX_PROFILE_FIELDS=8 \
 GTS_ACCOUNTS_REGISTRATION_BACKLOG_LIMIT=100 \
 GTS_ACCOUNTS_REGISTRATION_DAILY_LIMIT=50 \
 GTS_ACCOUNTS_REGISTRATION_OPEN=true \