Remove external dependency for the press-to-save bookmarklet

[benlk]

In #159 for #130 we revived the press-to-save Save Link functionality of this plugin through a WordPress official plugin that reimplemented functionality removed in WP 4.9.

That plugin hasn't received updates in 2 years.

We should copy the relevant code from that plugin to this plugin, and remove the dependency:

• the template bookmarklet code
• generation of the bookmarklet
• the bookmarklet handlers