Prepare for the first rubygems release of SolidusAdmin

[elia]

Summary

We're going to release solidus_admin as v0.0.x and tags will be namespaced as solidus_admin/0.0.x.
Releases will be handled manually using Bundler's helpers.

Once SolidusAdmin is ready to take over we'll link it to the main solidus version number and will jump directly to v4.x or v5 or whatever the version will be.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov]

Codecov Report

Merging #5394 (8695062) into nebulab/admin (689487b) will decrease coverage by 0.04%.
The diff coverage is 20.00%.

❗ Current head 8695062 differs from pull request most recent head 803ae90. Consider uploading reports for the commit 803ae90 to get more accurate results

@@                Coverage Diff                @@
##           nebulab/admin    #5394      +/-   ##
=================================================
- Coverage          88.80%   88.77%   -0.04%     
=================================================
  Files                609      609              
  Lines              14754    14758       +4     
=================================================
- Hits               13103    13101       -2     
- Misses              1651     1657       +6     

Files	Coverage Δ
admin/lib/solidus_admin/version.rb	0.00% <0.00%> (-100.00%)	⬇️
...ib/generators/solidus/install/install_generator.rb	49.63% <25.00%> (-0.75%)	⬇️

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