Alert rule validation does not accept valid templates

[kfindeisen]

OS: Ubuntu 64-bit 18.04 LTS
Chronograf version: 1.7.9
Installation method: built from source
Related issues: #5090

The alert rule GUI rejects the message template

CI ID = {{ with (index .Tags "ci_id") }}#{{ . }}{{ else }}Unknown{{ end }}

as invalid (specifically, it objects to the ., not the with block itself). I've confirmed by editing the corresponding TICKscript that this template does get properly evaluated.

It also rejects any call to len that does not appear inside an if or with statement, such as

# of Tags = {{ len .Tags }}

I've confirmed that len does work inside a condition, so this function doesn't seem to be specifically blocked.

I assume other built-in functions might also cause problems, but have not tested them exhaustively, or as part of complex expressions.