Currently, we shows data-no-recipients placeholder text even if
there are recipients selected. It's not visible cause it's value
is override by data-some-recipients helper text in case if there
are recipients selected.
Show data-no-recipients placeholder text only when `input` element
is first child of `pill-container`, which means there are no `pills`
selected by user yet.
@brockwhittaker wrote the original prototype for having
pills in the recipient box when users compose PMs (either
1:1 or huddle). The prototype was test deloyed on our
main realm for several weeks.
This commit includes all the original CSS and HTML from
the prototype.
After some things changed with the codebase after the initial
test deployment, I made the following changes:
* In prior commits I refactored out a module called
`user_pill.js` that implemented some common functions
against a more streamlined version of `input_pill.js`,
and this commit largely integrates with that.
* I made changes in a prior commit to handle Zephyr
semantics (emails don't get validated) and tested
this commit with zephyr.
* I fixed a reload bug by extracting code out to
`compose_pm_pill.js` and re-ordering some
calls to `initialize`.
There are still two flaws related to un-pill-ified text in the
input:
* We could be more aggressive about trying to pill-ify
emails when you blur or tab away.
* We only look at the pills when you send the message,
instead of complaining about the un-pill-ified text.
(Some folks may consider that a feature, but it's
probably surprising to others.)
