Add support for `via.external_link_mode`

[jon-betts]

To support LMS, there is an option via.external_link_mode which cases links to open in new tabs. Currently this is unsupported in viahtml.

To fix this we need to:

• Decode the param, and pass it on to the templates
• Enable the existing code from Via in the javascript to handle it

By copy pasting the code, I think we have the required functionality, it just isn't being enabled.

The values for this are:

• new-tab
• same-tab

But only new-tab actually matters, as any other value will cause the default situation

[jon-betts]

PR for this in: hypothesis/viahtml#9