Performance optimisation of `available_tags` method

We have a fairly good size redmine install with ~13,000 issues and we have grown to rely on this plugin -  about ~500 tags.  Unfortunately this plugin has become quite a significant bottleneck.

The issue seems to lie within this line:
```ruby
conditions << ids_scope.map{ |issue| issue.id }.push(-1)
```

The underlying SQL Rails generates is:-

```sql
SELECT `issues`.`id` AS t0_r0, `issues`.`tracker_id` AS t0_r1, `issues`.`project_id` AS t0_r2, `issues`.`subject` AS t0_r3, `issues`.`description` AS t0_r4, `issues`.`due_date` AS t0_r5, `issues`.`category_id` AS t0_r6, `issues`.`status_id` AS t0_r7, `issues`.`assigned_to_id` AS t0_r8, `issues`.`priority_id` AS t0_r9, `issues`.`fixed_version_id` AS t0_r10, `issues`.`author_id` AS t0_r11, `issues`.`lock_version` AS t0_r12, `issues`.`created_on` AS t0_r13, `issues`.`updated_on` AS t0_r14, `issues`.`start_date` AS t0_r15, `issues`.`done_ratio` AS t0_r16, `issues`.`estimated_hours` AS t0_r17, `issues`.`parent_id` AS t0_r18, `issues`.`root_id` AS t0_r19, `issues`.`lft` AS t0_r20, `issues`.`rgt` AS t0_r21, `issues`.`is_private` AS t0_r22, `issues`.`blocked` AS t0_r23, `issues`.`closed_on` AS t0_r24, `projects`.`id` AS t1_r0, `projects`.`name` AS t1_r1, `projects`.`description` AS t1_r2, `projects`.`homepage` AS t1_r3, `projects`.`is_public` AS t1_r4, `projects`.`parent_id` AS t1_r5, `projects`.`created_on` AS t1_r6, `projects`.`updated_on` AS t1_r7, `projects`.`identifier` AS t1_r8, `projects`.`status` AS t1_r9, `projects`.`lft` AS t1_r10, `projects`.`rgt` AS t1_r11, `projects`.`inherit_members` AS t1_r12 FROM `issues` LEFT OUTER JOIN `projects` ON `projects`.`id` = `issues`.`project_id` WHERE (projects.status <> 9 AND projects.id IN (SELECT em.project_id FROM enabled_modules em WHERE em.name='issue_tracking')) AND (projects.id=12)
```

Which, considering it only needs the `id`, is far more information that it really needs.

My initial thought was to swap out the line above with `pluck`:-

```ruby
conditions << ids_scope.pluck(:id).push(-1)
```

But I immediately ran into a Rails bug (https://github.com/rails/rails/issues/6132?source=cc, https://github.com/rails/rails/issues/8743?source=cc) within `to_sql`, which `pluck` makes use of.

Also `pluck` only fixes part of the performance issue - even using it `ids_scope.pluck(:id).push(-1)` on our redmine install would generate an array of 7505 ids.

I then thought instead of downloading all the ids into rails so that they can be converted into an SQL query to send back again to MySQL it would be better to pass the query that `ids_scope` would have generated in as a subquery, which is what I've done (plus a little hack to add in the bits of SQL rails forgets to add).

Before:
```bash
Rendered plugins/redmine_tags/app/views/issues/_tags_sidebar.html.erb (4328.5ms)
```

After:
```bash
Rendered plugins/redmine_tags/app/views/issues/_tags_sidebar.html.erb (831.2ms)
```

Thank you for an otherwise marvellous plugin,

Matt

(PS.  The commit "Way, way cleaner implementation" is in reference to my first attempt at addressing this, which was horrible :-) https://github.com/matthew-andrews/redmine_tags/commit/f6e49c795cd386a7bbcfa2ac2f8d5ba7e63f42aa)