Hide Save [Draft] button if post is published

[aduth]

Related: #2020 (comment)

This pull request seeks to omit the "Save" header button for published posts. Instead, the "Save" button is only offered as a means to save non-published posts with unsaved changes. A future revision will seek to add a "Revert to Draft" action.

Testing instructions:

Verify that Save is offered for drafts but not published posts:

1. Navigate to Gutenberg > New Post
2. Enter a title and/or content
3. Note that Save is shown
4. Press Publish
5. Note that Save is not shown
6. Make changes to the post
7. Note that Save is still not shown

[codecov-io]

Codecov Report

Merging #2058 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2058      +/-   ##
=========================================
+ Coverage   18.88%   18.9%   +0.01%     
=========================================
  Files         130     130              
  Lines        4204    4211       +7     
  Branches      716     718       +2     
=========================================
+ Hits          794     796       +2     
- Misses       2872    2875       +3     
- Partials      538     540       +2

Impacted Files	Coverage Δ
editor/header/saved-state/index.js	85.71% <100%> (ø)	⬆️
editor/utils/url.js	46.15% <0%> (-20.52%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fcc62a3...df0c5d4. Read the comment docs.

[jasmussen]

Looks good 👍 👍

This is a good start. As we finetune autosave, we might want to polish this further. For example, showing "Saved" still there (because it's true) and "Saving..." also when the Update button is pressed.