Add loading indicator when opening deeplinks for reader

[kean]

Fixes CMM-985: Loading a reader post on iOS by URL gives no user feedback

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 26.5 ❄️. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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[wpmobilebot]

📲 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	29980
	Version	PR #25028
	Bundle ID	org.wordpress.alpha
	Commit	466e86f
	Installation URL	546ab22ve45bo

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[wpmobilebot]

📲 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	29980
	Version	PR #25028
	Bundle ID	com.jetpack.alpha
	Commit	466e86f
	Installation URL	6eg5nttkb8ng8

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud