Add: Ability to process already created content

[dinhtungdu]

Description of the Change

Add bulk and row actions to classify supported post types.
Add a new class: Classifai\Admin\BulkActions to manage bulk actions, not only
for posts classification, but also images.
Use the SavePostHandler::classify() method to classify posts.

Alternate Designs

n/a

Benefits

Provide UI for classifying supported post types in bulk.

Possible Drawbacks

n/a

Verification Process

1. Allow post/page to be classified.
2. Go to Posts list table.
3. Select some posts (Choose posts that haven't classified yet).
4. Choose Classify from bulk actions select box.
5. Click Apply.
6. See posts classified and a success notice.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Closes #121. Relate: #129.

[ryanwelcher]

@dinhtungdu the bulk action worked locally but when clicking on the individual Classify links in the row actions, I was getting a 404 page.

It looks like the URL is incorrectly formatted and extra items are being appended to the end:

Example:

http://classifai.test/wp-admin/edit.php&action=classify&ids=6?_wpnonce=fd5cf6de48

[ryanwelcher]

Overall the code looks good. I did run into an issue with the Classify link in the row actions.

[ryanwelcher]

@dinhtungdu changes work as expected. I found an edge-case when testing locally, however.

Steps to reproduce:

1. Go to Posts list
2. Quick Edit a post
3. Save
4. Click on the Classify link

Expected results: Post is rescanned
Actual Results: I was redirected to the Dashboard.

[dinhtungdu]

@ryanwelcher I fixed that case, this PR is ready to review again