Support custom subject text

[czy-29]

Close #230

[Enet4]

The feature works nicely in the general case. I did notice that there are no constraints to the subject content in terms of length and characters. Considering that any client can request these on the spot, it might be for the best that we add some safeguards (e.g. 100 maximum length).

Also, subject is ignored if style is given an unsupported value.

[czy-29]

The feature works nicely in the general case. I did notice that there are no constraints to the subject content in terms of length and characters. Considering that any client can request these on the spot, it might be for the best that we add some safeguards (e.g. 100 maximum length).

What do you think we should do when the length exceeds the limit? Should we treat it as if there is no subject parameter?

[czy-29]

Also, subject is ignored if style is given an unsupported value.

As for this issue, it seems to be an existing bug. Consider the following two requests:

• https://deps.rs/crate/syn/2.0.66/status.svg?style=for-the-badge&compact=invalid
• https://deps.rs/crate/syn/2.0.66/status.svg?style=invalid&compact=true

They each have one parameter that is invalid, but neither of them responds to the other valid parameter.
The root of this problem seems to be that we can't simply get the extra_config through: serde_urlencoded::from_str::<ExtraConfigPartial>(qs).
We need to think about how to fix this.

[czy-29]

@Enet4
All the bugs have been gracefully fixed! The relevant rationales are written in the code comments.
The fix was a bit trickier than expected, but eventually I managed to find a graceful solution.
Looking forward to your review!