Mattermost-server: Position the autocomplete popover relative to the @, ~, or / trigger in the post draft

[mattermod]

In addition to the work recently done on the autocomplete UI, we'd like to position the autocomplete popover in relation to the @, or / trigger within the post draft.

Behaviour notes:

1. If the trigger character in the post draft is positioned on the far left side of the post draft input, position the autocomplete popover with the left edge of the post draft.

1. Position the left edge of the autocomplete popover so that the icon or avatar is aligned with the aligned horizontally with the @, or / trigger

1. If the trigger character is positioned at an x value that is greater than the autocomplete popover's width subtracted from the post draft width, align the autocomplete popover to the right edge of the post draft input

Note 1: If there are multiple lines of text, the autocomplete should float above the line the cursor is on.
Note 2: We want the same behaviour for the autocomplete suggestion box in the right-hand side thread draft box and the edit post modal

Related to suggestion box behaviour here: https://mattermost.atlassian.net/browse/MM-29082

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JIRA: https://mattermost.atlassian.net/browse/MM-24072

[pablovelezvidal]

Hi! I would like to take this one!