Add optional text input for link posts

[rleed]

To resolve #504.

[ekzyis]

After looking at these changes, I realized that it should be a lot easier to just also set text and not only url of the item instead of creating a comment as a separate item. This has the following advantages:

• can reuse existing create_item postgres function, so no db migration with support for noCosts needed
• fee escalation can work as before: no need to mention something like free, but compounds any future spam penalty which not every user might understand
• it just works™ without any backend changes 🎉 (did not test if it really requires zero backend changes but i have good faith belief in that)
• consistent edit UX: you can edit everything you were able to create in the same form

Another thing is that it doesn't "smell good" to add noCosts as an exception to create_item (which was meant to be a generic function) for a single use case.

It also might be better UX if the provided summary actually shows up at the top for every user instead of as a separate comment which might get lost if there are many comments? Using item.text kind of works like a pinned comment in that case (except it's just regular UX like for discussion posts)

Thanks for putting in the work! It wasn't clear to me before that creating a separate comment might lead to all of this.

[ekzyis]

Thinking more about this, I wondered if it would break some assumptions in our code if link posts now suddenly also can have text set to something - like code where we want to distinguish between link posts and discussion posts.

However, searching for item.text didn't look like there is anything that would break in that case.

[huumn]

I agree that this would ideally not require special treatment and simply occupy the text column on the post.

[rleed]

Great idea to simplify with the text field... I will pursue that!

[rleed]

I think this is ready now.