Show Reader profiles for post mentions

[crazytonyli]

Fixes https://linear.app/a8c/issue/CMM-2158.

Tapping at-mentions now shows the RreaderUserProfileView sheet, which is the same as tapping comment author avatars.

For the sake of keeping the implementation simple, the solution is resolving the <a>@username</a> links to a user profile object when viewing the post. The expectations are 1) there will not be many mentions in regular posts and 2) it'd be pretty quick to resolve them, at least they should be resolved before user reading the post and tapping a mention link.

[dangermattic]

	2 Warnings
⚠️	View files have been modified, but no screenshot or video is included in the pull request. Consider adding some for clarity.
⚠️	This PR is larger than 500 lines of changes. Please consider splitting it into smaller PRs for easier and faster reviews.

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📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	33181
	Version	PR #25793
	Bundle ID	org.wordpress.alpha
	Commit	14b2ee0
	Installation URL	7dkelhs606ra8

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📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	33181
	Version	PR #25793
	Bundle ID	com.jetpack.alpha
	Commit	14b2ee0
	Installation URL	55r8k3ki03cjg

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[wpmobilebot]

Version 27.1 has now entered code-freeze, so the milestone of this PR has been updated to 27.2.