[feature] Hashtag federation (in/out), hashtag client API endpoints (#2032)

* update go-fed

* do the things

* remove unused columns from tags

* update to latest lingo from main

* further tag shenanigans

* serve stub page at tag endpoint

* we did it lads

* tests, oh tests, ohhh tests, oh tests (doo doo doo doo)

* swagger docs

* document hashtag usage + federation

* instanceGet

* don't bother parsing tag href

* rename whereStartsWith -> whereStartsLike

* remove GetOrCreateTag

* dont cache status tag timelineability

internal/db/bundb/migrations/20230718161520_hashtaggery.go

@@ -0,0 +1,76 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package migrations
+
+import (
+	"context"
+
+	"github.com/uptrace/bun"
+)
+
+func init() {
+	up := func(ctx context.Context, db *bun.DB) error {
+		return db.RunInTx(ctx, nil, func(ctx context.Context, tx bun.Tx) error {
+			// Drop now unused columns from tags table.
+			for _, column := range []string{
+				"url",
+				"first_seen_from_account_id",
+				"last_status_at",
+			} {
+				if _, err := tx.
+					NewDropColumn().
+					Table("tags").
+					Column(column).
+					Exec(ctx); err != nil {
+					return err
+				}
+			}
+
+			// Index status_to_tags table properly.
+			for index, columns := range map[string][]string{
+				// Index for tag timeline paging.
+				"status_to_tags_tag_timeline_idx": {"tag_id", "status_id"},
+				// These indexes were only implicit
+				// before, make them explicit now.
+				"status_to_tags_tag_id_idx":    {"tag_id"},
+				"status_to_tags_status_id_idx": {"status_id"},
+			} {
+				if _, err := tx.
+					NewCreateIndex().
+					Table("status_to_tags").
+					Index(index).
+					Column(columns...).
+					Exec(ctx); err != nil {
+					return err
+				}
+			}
+
+			return nil
+		})
+	}
+
+	down := func(ctx context.Context, db *bun.DB) error {
+		return db.RunInTx(ctx, nil, func(ctx context.Context, tx bun.Tx) error {
+			return nil
+		})
+	}
+
+	if err := Migrations.Register(up, down); err != nil {
+		panic(err)
+	}
+}