Style Upload picture buttons

[Sameerkash]

The < input> file tags have no styling and make it look odd compared to the rest of the app. It'd be great if they have some styling. Although this is a bit tricky, I did it in the last sprint, so I can accomplish this with a nice-looking upload button and an image preview in the Post option.

If this sounds good, please assign this to me, I'll get the PR up soon

cc @aitikgupta , @bodhisha