Communication: Users can save a single post multiple times

[PaRangger]

Describe the bug

Currently, it is possible to send multiple requests in quick succession to save a post multiple times, leading to a state that causes a error 500 in the "Saved Posts" section. While the server checks if an entry exists already, in case of two requests happening at the same time and querying the database simultaneously, it will create 2 saved post entries.

To Reproduce

1. Go to course
2. Go to the communication tab
3. Click a channel
4. Write a post
5. Click the save post button quickly a lot of times
6. Go to "In Progress" section of saved posts
7. Notice that the post appears multiple times
8. When trying remove the saved post you should receive an "Interal Server Error" notification

Expected behavior

It should not be possible to save a post multiple times.

Screenshots

Which version of Artemis are you seeing the problem on?

7.9.1

What browsers are you seeing the problem on?

Other (specify in "Additional context")

Additional context

No response

Relevant log output