Updated prepare_date_terms to only call date_i18n once

[Rahmon]

This is a "take over" of #1958 just to address some minor things.

Description of the Change

Calling date_i18n a lot gets pretty expensive. Only calling it once for each prepare_date_terms call is much more efficient and make indexing faster.

Props @WPprodigy
cc: @rinatkhaziev @nickdaugherty @pschoffer @netsuso @parkcityj

Alternate Designs

N/A

Benefits

Removes extraneous calls to date_i18n and makes indexing faster.

Possible Drawbacks

N/A

Verification Process

There are 4 calls to prepare_date_terms() per post being indexed. Each call resulted in 12 calls to date_i18n(), which ends up going down a bit of a rabbit hole and eventually adding up to a lot of time spent doing filters and alloption calls. So per page of 500 posts being indexed, that is 24,000 calls (500 x 12*4)- seen in cachegrind.

Before the changes, a test group of 500 posts took 7.91601 seconds running prepare_date(). After this PR, 0.85888 seconds.

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: #1958

Changelog Entry

Changed prepare_date_terms to only call date_i18n once. Props @WPprodigy

[felipeelia]

@Rahmon I forgot to mention but there are some warnings in PHPCS that should be addressed as well. Thanks! actually, nevermind. They seem to come from the current codebase. Sorry.