Add option for post roll for timed timelapse to duplicate last frame

[dattas]

What does this PR do and why is it necessary?

Adds ability to simply repeat the last frame of capture instead of capturing several minutes (or hours) of footage after print is completed.

How was it tested? How can it be tested by the reviewer?

Tested on 3 octoprint instances with 3 different cameras and some different plugins as well. It can be tested via going to timelapse, unchecking the new checkbox (ensuring there is a postroll interval) and saving, then running a print.

Any background context you want to provide?

What are the relevant tickets if any?

#1422

Screenshots (if appropriate)

Further notes

[foosel]

Hmm, what I don't like here is the code duplication going on. ZTimelapse and TimedTimelapse with non captured post roll now basically contain the exact same code in process_postroll. That screams for extracting that into a custom "copying_postroll" (or better named) method on the Timelapse parent class and then just calling that from the child classes were appropriate.

Could you modify your pr accordingly? No problem if not, would just mean merging would have to wait until I'm back on my feet (which I'm currently not).

Cheers!