Expose more video transcoding options and add related documentation

[mgogoulos]

Describe the feature you'd like
After discussing with Yaroslav on #232 I realize that

• a few options related to video transcoding could be exposed on the admin interface, for example ffmpeg presets
• video transcoding related documentation needs be added (on the Administrator section of the docs)

[slhck]

You might want to have a look at https://github.com/svt/encore as either

• an example on what kind of options are useful to expose, or
• potentially simply reusing this as a backend in the long run

At the very least, I think it would be good to have the users be able to select:

• The resolutions that are being encoded
• Video and audio bitrates for different codecs/resolutions (i.e. defining the "bitrate ladder")
• Additional options applied to videos such as video filters
• The type of scaler used for the scale command