[feature] Allow users to set custom css for their profiles + threads (#808)

* add custom css account property + db func to fetch

* allow account to get/set custom css

* serve custom css for an account

* go fmt

* use monospace for customcss, add link

* add custom css to account cache

* fix broken field

* add custom css docs to user guide

* add `accounts-allow-custom-css` config flag

* add allow custom css to /api/v1/instance response

* only show/set custom css if allowed to do so

* only set/serve custom account css if enabled

* update swagger docs

* chain promise

* make bool a bit clearer

* use cache for GetAccountCustomCSSByUsername

internal/typeutils/internaltofrontend.go

@@ -197,6 +197,7 @@ func (c *converter) AccountToAPIAccountPublic(ctx context.Context, a *gtsmodel.A
 		Emojis:         emojis, // TODO: implement this
 		Fields:         fields,
 		Suspended:      suspended,
+		CustomCSS:      a.CustomCSS,
 	}
 
 	c.ensureAvatar(accountFrontend)
@@ -678,6 +679,9 @@ func (c *converter) InstanceToAPIInstance(ctx context.Context, i *gtsmodel.Insta
 				MinExpiration:          instancePollsMinExpiration, // seconds
 				MaxExpiration:          instancePollsMaxExpiration, // seconds
 			},
+			Accounts: &model.InstanceConfigurationAccounts{
+				AllowCustomCSS: config.GetAccountsAllowCustomCSS(),
+			},
 		}
 	}