zapier convert: Replace variables in URLs (PDE-10)

[eliangcs]

Developers can put variables from in a URL, such as https://{{account}}.example.com/{{resource_name}}. This PR makes sure we replace those variables in URLs before we send requests.

Example app to try out: 83342:

zapier convert 83342 replace-vars
cd replace-vars
npm install
zapier test

Expect zapier test to fail on a TypeError for test case Full Search and Full Trigger due to a bug in core, which was addressed in zapier/zapier-platform-core#59.

[ZaneLyon]

I'm testing the Jira integration, so I added a Jira task reference to the title of this PR to see if linking is working properly.

[xavdid]

sweet, that'll be really convenient.

Only question I had was about the reason for removing the {} argument in all those calls to replaceVars? Seems like it wouldn't do anything in that function while empty (which could be why you're removing it)

[eliangcs]

@xavdid yeah, you guess it right. You get the same result whether you pass a {} or anundefined to replaceVars, so I removed it to make the code cleaner.