Currently, when the saved snippets are added or removed the changes
are not properly shown across different browser tabs if the dropdown is
open. This commit fixes this.
This error occurred when opening the saved snippets dropdown while
editing a message caused by `get_offset_to_window()` expecting a
single JQuery element in the selected set. This function was used on
`$(this.dropdown_input_visible_selector)` which is a class selector
and can return multiple elements when multiple saved snippets button
are rendered.
This commit resolves this bug by creating dropdown widgets with unique
`widget_selector` for each saved snippets button.
Renames "Add a new saved snippet" to "Create a new saved snippet" and
"Add saved snippet" tooltip to "Insert saved snippet".
Moves "Create a new saved snippet" button to the bottom sticky area of the
dropdown.
This is follow-up of #31359 that moves "Saved snippets" button from
"send_later" popover to message formatting buttons and adds support
to use saved snippets in message-edit UI.
Fixes#31830.
