[MM-23414] Fix up multiselect styling

[fmunshi]

Summary

• The original issue was just that the dark mode styles were too dark - the cause of which i traced back to MM-22873 - Updating input UI in DM modal #5011
• Turns out the select input of this modal never modified its colours to match the current theme so I made some changes to its styles to look good in both dark and light themes
• A lot of the styles were borrowed heavily from /components/widgets/inputs/user_emails_input.scss (from the invite team members select list)

Ticket Link

https://mattermost.atlassian.net/browse/MM-23414

Before

After

[asaadmahmood]

@fm2munsh I submitted a PR to fix the dark theme issue and the height.
I think we can still take the rounded thing and the stylistic changes from this PR.
See if you can resolve the conflicts.
#5079

[asaadmahmood]

See comment. And I think we can keep the react-select class and react-select-auto class.

[jwilander]

LGTM

[fmunshi]

@asaadmahmood Thanks - I didnt realize you already had a fix for this - I merged my changes with yours and kept the styling changes I made

[asaadmahmood]

Awesome, thank you.

[lindalumitchell]

Tested with the four in-app default themes, two of the custom themes from https://docs.mattermost.com/help/settings/theme-colors.html#custom-theme-examples, and using the color pickers to create a new custom theme. No issues found on that selector or others that I looked at, including the invite people UI as mentioned above. LGTM!

[mattermod]

Test server destroyed

[mattermod]

Will try to auto merge this PR once all tests and checks are passing. This might take up to an hour.

[mattermod]

@fm2munsh
Error trying doing the automated Cherry picking. Please do this manually

+++ Updating remotes...
Fetching upstream
Fetching origin
+++ Updating remotes done...
+++ Creating local branch automated-cherry-pick-of-#5087-upstream-release-5.22-1585151061
Branch 'automated-cherry-pick-of-#5087-upstream-release-5.22-1585151061' set up to track remote branch 'release-5.22' from 'upstream'.
+++ Downloading patch to /tmp/5087.patch (in case you need to do this again)

+++ About to attempt cherry pick of PR. To reattempt:
  $ git am -3 /tmp/5087.patch

Applying: Improve direct messages multiselect styling
Using index info to reconstruct a base tree...
M	components/multiselect/__snapshots__/multiselect.test.tsx.snap
M	components/multiselect/multiselect.tsx
Falling back to patching base and 3-way merge...
Auto-merging components/multiselect/multiselect.tsx
CONFLICT (content): Merge conflict in components/multiselect/multiselect.tsx
Auto-merging components/multiselect/__snapshots__/multiselect.test.tsx.snap
CONFLICT (content): Merge conflict in components/multiselect/__snapshots__/multiselect.test.tsx.snap
Patch failed at 0001 Improve direct messages multiselect styling
Use 'git am --show-current-patch' to see the failed patch
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

+++ Conflicts detected:

UU components/multiselect/__snapshots__/multiselect.test.tsx.snap
UU components/multiselect/multiselect.tsx

+++ Aborting in-progress git am.

+++ Returning you to the master branch and cleaning up.