Change Notice component to be in-line with Photon spec and check its colours are correct.

[tofumatt]

As mentioned in mozilla/addons-frontend#4548, it seems:

1. Our Notice component needs style tweaks to be in-line with Photon
2. The Photon styles are wrong re: "generic" colours for buttons (they say blue but look grey)
3. We should allow Notice components to more easily render buttons that are links
4. The fix for fix: Make "Download Firefox" notice less red addons-frontend#4548 should be changed to be a button link to "Download Firefox"

[kumar303]

I asked for clarification in FirefoxUX/photon#287 . I'm pretty sure they should be gray.

[kumar303]

I'm like 99% sure these buttons should be gray so I'm making them gray. The blue button is too much of a call to action on the page.

[ioanarusiczki]

@kumar303 I checked on AMO dev (Win10) for the filter but I do not see it displayed as shown here mozilla/addons-frontend#7146

AMO dev

[ioanarusiczki]

@kumar303 Ok, I figured out how the button becomes visible. The color is grey and it can be clicked.
It could be a darker grey (in my opinion) but while hovering it changes it's contrast.
Verified on AMO dev with FF63(Win10 & Android 8.0)

The Download Firefox button mentioned here is green but it's been like that for a while and the message from mozilla/addons-frontend#4548 was removed (that was for other browsers)