Mattermost-server: Remove duplicate id uses by the search container

[mattermod]

This issue was found by the aXe Chrome extension for finding WCAG issues. In order to successfully complete this issue, you'll need to make sure to test using the tool and ensure that all of the warnings covering the components scoped below are cleared. You can find more information on the issue at the link below:

We are commonly using the same id tag on html elements repeatedly. We need to make sure we are using unique ones throughout the application. Some of these id attributes are also used by Cypress, if you are purely removing them you’ll need to correct the Cypress tests accordingly.

The following ids are used by the search bar container, and are duplicated when the RHS is open, regardless of the visibility:
searchBox
channelHeaderMentionButton
channelHeaderFlagButton
searchbar-help-popup
sidebarCollapse
searchFormContainer
searchClearButton

Using the aXe tool, find all instances of the above error, and correct them by:

1. Changing the id to something else more unique OR
2. Removing the id if it is unnecessary.

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JIRA: https://mattermost.atlassian.net/browse/MM-20269

[allenlai18]

can i work on this?

[allenlai18]

https://github.com/mattermost/mattermost-webapp/pull/4431