Allow filling alert details template with HTML fields/tags

[SuddenSelect]

Alerts' details (email body) can currently be enhanced by using HTML tags, however embedding there Field or Tag value (ex. {{ index .Fields "someField" }} ) automatically triggers html escaping on said value (as per example here)
While this is a sensible default behavior, it unnecessarily limits Users flexibility when creating HTML-rich email notifications, and from a User's perspective seems counter-intuitive - some email body elements can have HTML and some cannot.

Proposed solution introduces a new function to template function map for encapsulation of given string in a type html/template/HTML as suggested here.

Added tests are based on TestStream_AlertEmail.

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[SuddenSelect]

2 out of 3 containers in CircleCI job seem to time-out on apt-get setup