Modified empty_subscription_feed_subtitle to improve discoverability of the subscription import/export menu

[Poslovitch]

What is it?

• Bugfix (user facing)
• Feature (user facing)
• Codebase improvement (dev facing)
• Meta improvement to the project (dev facing)

Description of the changes in your PR

• Changed the default value for empty_subscription_feed_subtitle string key
• Removed all translations for this key (as requested in Ensure users find new location of subscription import/export #8508)

Before/After Screenshots/Screen Record

• Before:

- After:

Fixes the following issue(s)

• Fixes Discoverability of the subscription import/export menu #8504

APK testing

The APK can be found by going to the "Checks" tab below the title. On the left pane, click on "CI", scroll down to "artifacts" and click "app" to download the zip file which contains the debug APK of this PR.

Due diligence

• I read the contribution guidelines.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[litetex]

I think we need a few changes:

1. Somehow "empty_subscription_feed_subtitle" is used inside list_empty_view.xml which is used nearly everywhere and not just in the subscription area:
   
2. The layout is not correctly aligned
3. See the comment below

[foss-enjoyer]

That's way too long. "Import & export subscriptions from 3-dot menu." would fit better.

[Poslovitch]

I'm getting back on working on this PR. Please pardon the long delay.
It's currently untested and still WIP.

[Stypox]

Since I couldn't push to this PR (you probably forbade pushes to this branch by maintainers), I opened #9501 instead. Thanks anyway!