[ui] Add loading state for button creation and export csv

[johnkuney]

Related to #903

Description

Adding some loading state when creating a button or exporting a csv. Just went for the basic "Loading..." Could potentially do a spinner/animation if we want something fancier

Test plan

preview the site, and try making a button with several addresses and exporting a csv. Should see the button change to a loading state

[Klakurka]

IMO may as well go for the the actual spinner for this one.

• Change the text to a spinner.
• Make sure the button width doesn't change (it currently gets wider to accomodate the "Downloading..." text).
• Once #926 is done, we'll want to make sure this is working for that one too so this can be merged after that one.

[johnkuney]

Okay for sure, added. I'm realizing the button code/styles in general throughout the app is a bit unkempt...I'd like to consolidate everything and clean it up, but can do that in another PR

[chedieck]

Code looks good, works well.

Nonetheless, maybe something for another PR, but both the download of the CSV and the paybutton creation don't need the user to wait in the page, for the job to complete.

So maybe in the button creation we could have it just close the dialog and go to the button view (I was implementing this but left it aside for some other priorities)

and in the CSV we could show a message that goes away, something like: "Your download will start in a moment..."

[Klakurka]

Good point @chedieck - let's do that in a follow up. This is at least a bit better than what's in master.