Refactor the share buttons template

The issue

The template used for rendering of the share buttons goes against DRY principle, which make it harder to maintain and actually harder to override/customize it. Hard-coded URLs are also a bit of an issue.

Consider following cases...

• In a hypothetical case where the number of supported social outlets grow you'd have to add more and more repeating code into the template.
• When a custom button has to be added - you'd have to override and maintain the whole template in your theme
• In a case where a share URL needs to be changed, and the template is overridden in your theme - besides the module update this will require updating the overridden template as well

Proposed resolution

- create general button template,
- refactor main template to include button template as needed (per what is available in exposed variables)
- move URL formatting into preprocess or specialty callback