User is able creating a PD without being a recipient

[PatrickRoethlisberger]

Bug Report

Current Behavior
A user is able to create a private discussion without being a recipient when he deletes himself in the initial recipient's selection.

Steps to Reproduce

1. Create a new PD
2. Remove own user from recipients
3. Post discission

Expected Behavior
The user is prevented from creating PDs that he is not part of.

Environment

• Flarum version: 1.0.4
• Extension version: 1.1.1

Possible solution(s)
Prevent user from removing himself/ display a warning when a user tries to delete himself that he's not able adding himself back again

Additional Context
Posts from discussion which user 6 created and invited users 2 and 4 while removing himself:

DB [flarum]> select id,user_id,type,content from posts where discussion_id = 123;
+-------+---------+--------------------+--------------------------------------------------------------------+
| id    | user_id | type               | content                                                            |
+-------+---------+--------------------+--------------------------------------------------------------------+
|  1234 |       6 | comment            | <t><p>Content</p></t>                                              |
|  1235 |       6 | recipientsModified | {"new":{"users":[2,4],"groups":[]},"old":{"users":[],"groups":[]}} |
+-------+---------+--------------------+--------------------------------------------------------------------+