Small side panel formatting fix: hide button <p> containers instead of buttons; fix prev/next frame animation buttons

[fedarko]

This doesn't entirely fix the problems brought up in #513, but it is a start -- it removes some unnecessary whitespace in the sample/feature metadata coloring and animation panels.

Hiding the button(s) inside a <p> tag still leaves some extra whitespace, since empty <p> tags still take up some space. Hiding the <p> tag directly (instead of the button(s)) fixes this, so now for example the default state of the animation panel looks a bit nicer:

previous	now

This PR also adjusts the animation panel so that:

• disabled buttons (prev frame / next frame, when on the first/last frame) are now given a special style to indicate they're disabled
• the next frame button is correctly disabled when on the last frame. It was off by 1 before, apparently.

prev frame button disabled	next frame button disabled

[emperor-helper]

The following artifacts were built for this PR: empire-biplot.qzv, empire.qzv, empress-tree.qzv, just-fm.qzv, plain.qzv

[ElDeveloper]

@fedarko can you pull from upstream to see the mchelper build?

[ElDeveloper]

I added/removed a suggestion so that triggered a new build.

[ElDeveloper]

Sounds fair. Thanks @fedarko !