Support update

[Qup42]

Could also be made to work with the current setup of having the access token as a query parameter.
Currently depends on ad-freiburg/qlever#1691, but it could also be done without this PR by adding the access token to the parameters.

Closes #122.

[hannahbast]

1-1 with Julian, looks great and works, thanks a lot!

It would be great to show a meaningful message (like: "Update successful") when the update was successful (no need to display the JSON for this PR).

[hannahbast]

1-1 with Julian, this looks great and works. A few suggestions for improvement:

1. Remove the extra space before the "Automatically add names ..."
2. Add button for "Clear delta triples" and "Clear cache complete"
3. Add some more information to the "Update successful" message

NOTE: There already is a field for the access token (called API token) in the backend config as well as buttons for clear cache etc.

[Qup42]

2. The Clear delta triples button currently is reported as having had an error, because the response is not JSON but plain text. I'll change the response the response to JSON in QLever.

The whole warmup functionality is broken quite much (see #130). Fixing that that is out of scope, so I won't add a corresponding Clear delta triples button in the Django Admin in this PR:

[hannahbast]

Thorough 1-1 with Julian, this is great and works fine now. A few more glitches:

1. Functionality of the new buttons: when successful after the red x, the spinning wheel icon disappears completely
2. When buttons are displayed the whole column moves to the left; more padding
3. Number of inserted triples is 0 although it isn't (maybe same for delete triples)