Set widget BASE_TARGET on ENV variables

[hcarreras]

It should be possible to decide where the widget's links should be open.

The PR makes it possible to set an ENV variable at build time to open the links in a new tab by setting BASE_TARGET to _blank.

[CLAassistant]

All committers have signed the CLA.

[hrithikt]

@hcarreras This looks useful.

However, wouldn't it be better to add this as an option under chatbot customizations? This way, users can choose whether the links should open in the parent window or in a new tab for each chatbot.

[hcarreras]

I agree that'd be even better! I'm a bit busy these days, so I'm unsure when/if I'll be able to make the changes. Since this is an improvement over what's there already, what would you say about merging this and creating an issue request with the improvement?