[Master feature] Support RTL for amp-story

[newmuis]

Right now, everything is designed as LTR. This is a cover bug for enabling RTL support. We should consider the story to be RTL if:

1. Any ancestor of the story element has its dir attribute set to "rtl"
2. Failing that, the direction property of its computed style is "rtl"
3. Failing that, document.dir is set to "rtl"

If none of these are true, we should consider the story as LTR. We can calculate this once per story, at build time.

• Navigation areas of the screen should be flipped (i.e. tapping the left side of the screen will advance to the next page; the right side of the screen will go back to previous)
• Keyboard keys should be flipped (left arrow for next; right arrow for previous)
• System layer UI elements should be flipped (the buttons normally in the top right should be in the top left, and vice versa), on desktop and mobile
• Bookend UI elements should be flipped
• amp-story-consent UI elements should be flipped
• Share menu UI elements should be flipped
• Info dialog UI elements should be flipped
• Hint layer UI elements should be flipped
• Any icons indicating directionality based on reading direction (as opposed to, say, handedness) should be flipped

[aghassemi]

@newmuis @choumx Could we consider move this to P1/P2? Another market this has come up is Japan.

[morsssss]

This is harder no doubt... but would it be worth creating another issue to enable vertical instead of horizontal? At least one major language group expects that :)

[newmuis]

The visual diff tests added in #17110 show that many of the templated UIs need some love in RTL:

The share menu actually looks okay:

[Enriqe]

Yep, the share menu was just fixed in #17098 alongside the system layer. I am working on the remaining templated UIs. Thanks for including RTL in the visual diffs!