[share url] fixes buttons on share panel using dark theme

[BigFunger]

fixes #6270

adds btn and btn-default classes to the buttons on this page.

[BigFunger]

Slight issue... I fixed this by adding the btn and btn-default classes to those buttons.
This changes the appearance of the buttons when the dark theme is not enabled, however it does make them look like the rest of the default buttons in the app with the exception of the search button in the nav bar. The navbar button has custom styling though. Thoughts?

Old Appearance

New Appearance

[BigFunger]

@alt74 Your input would be valuable as well.

[spalger]

This is how buttons should look by default, based on the theme, and I'm fine with that. If we want standard buttons to look different we can have a larger discussions about changing btn-default

[spalger]

LGTM

[alt74]

buttons in "new appearance" screenshot look better to me compared to old. also, it feels more natural that the action button next to a field is clearly darker than the field.