Add ability to support google manager tracker

[jtapia]

What it does?

• Add ability to specify what kind of tracker do you want to use on the admin
• Add script for google manager
• Remove unnecessary columns
• Add tracker_type to spree_trackers DB
• Refactor current method
• Add and fix test suite

Preview

[fastjames]

@jtapia Would you mind rebasing this? We are swinging back to some extensions with open PRs, and we would like to merge this one (this work is relevant to our interests too).

[fastjames]

Looks good to me!

[jacobherrington]

Thanks @jtapia! This looks okay to me, other than the odd indentation.

Do you mind taking a look @kennyadsl?

[kennyadsl]

Having two active trackers is a requirement? If not I think we should try to don’t change the .current API and set the type of existing trackers via the default value of the migration that add the type column. What do you think?

[jtapia]

@kennyadsl yes, that's the main idea, to have multiple tracking services at the same time, I made this 3376cb2 to avoid iteration on the views and to avoid changes on self.current method, what do you think?

[kennyadsl]

Left some comments. Also, did we have something that auto-mark old trackers as non-active when you activate a new one? Do you think we could need something similar? Maybe into another PR?

[jtapia]

@kennyadsl once this is merge, I'll bring these changes jtapia@488dad5 to include facebook pixel

[kennyadsl]

There is one thing I can't get: this PR is adding the main script for Tag Manager which, as far as I know, only defines a DataLayer you should push events on but right now no events has been pushed so I'm not sure what it's tracking. Do we at least need to support the pageView event? How does it work tracking other actions (like purchase) for solidus_trackers users?

[jtapia]

@kennyadsl right, here is the change for that 907fc64#diff-ef80dbd5bb074228fe38463e26a993f6R17