Rename endpoint to entrypoint

Problem/Motivation

We use the term endpoint to refer to the https://example.com/jsonapi address. According to @gabesullice on Drupal Slack #contenta channel on October 15th 2019, a more correct term for this is the entrypoint.

Proposed resolution

Propose to change this in all places *except* the config schema. Once we implement #3088635: Allow multiple sources we will have to change the config schema anyway, so suggest deferring changing the name of the config key until then.

Remaining tasks

All the things

User interface changes

Reference to Endpoint on the module's config page will change to EntryPoint.

API changes

None

Data model changes

None

Release notes snippet

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