Remove explicit Dashboard setting

[newhinton]

I have experimented with removing the dashboard setting.

I found out if we use css-selectors, we can specifically target the dasboard-'default' setting which is the same as the login background. This allows us to directly target that selection, instead of breaking the customization provided by nextcloud and moving that setting into the personal setting.

@jancborchardt What do you think about making this setting implicit? It allows us to remove the dasboard setting from the personal and admin pages.

(If you want to check it out, it is in the dev-branch)