Gallery should use the new viewer

[skjnldsv]

Since 16 we implemented a new viewer.

• Gallery needs to display the same videos and photo mimes
• Gallery needs to open the viewer on click

Fix #473

[szaimen]

@skjnldsv Is this now getting developed since nextcloud/viewer#271 is merged? :)

[skjnldsv]

@szaimen yes

[XeonG]

got a folder of webp...and no thumbnails... how is this not already supported... its 2020 guys

[sbrl]

Off-topic, but @XeonG according to caniuse ~80% of users have webp support currently.
On a side note, I can't remember why but I encountered an issue generating webp thumbnails myself and had to disable it.

At any rate, that should be a separate issue @XeonG.