luci-app-ustreamer: complete rewrite

luci-app-ustreamer: complete rewrite

New features:
- Implement all supported ustreamer features
- Detailed UI text and description based on the program help
- Input validation for all parameters
- Stream preview with link to the stream page
- Dark theme colours for the stream preview
- Bulgarian translation (complete)

Bug fixes:
- Use of poll.add inside the render function results in a fork bomb
- Repeated use if setTimeout results in a fork bomb when the stream
is not available (old bug from luci-app-mjpg-streamer)

Merge:
- I tried to keep existing translations as much as possible
- All existing features, except [video_devs]

Removed:
- [video_devs] parameters, this or a similar feature will be implemented
once I fully test it, and choose an optimal strategy, with support
for multiple video input devices. In order to comlpete work on this
feature, I need programatic access to the configuration name for
each instance:

config ustreamer 'configuration_name'

Formatting:
- Format code for readability and to fit 80 column where possible

Notes:
The values for image control varies between camera models, therefore
the range is unrestricted.

Due to a race condition, two instances of the package got created.
I put a lot of effort and testing in every single detail, and the
other implementation got merged first. All features and translations
are merged here, except for [video_devs], which will be reworked
later.

Signed-off-by: Georgi Valkov <redacted>
Closes #8324
Link: https://github.com/openwrt/luci/pull/8324/
Signed-off-by: Paul Donald <redacted>