Add slack param to disable link unfurling

[cezarsa]

This PR adds a new parameter to the Slack service to allow disabling link unfurl. By default, Slack will try to unfurl links present in the notification message. However, in most cases, this is not useful as Argo URLs are usually private and inaccessible from Slack servers.

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (7b19d2c) 55.16% compared to head (29ba9ce) 55.08%. Report is 1 commits behind head on master.

:exclamation: Current head 29ba9ce differs from pull request most recent head bbfca43. Consider uploading reports for the commit bbfca43 to get more accurate results

Files	Patch %	Lines
pkg/services/slack.go	0.00%	2 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #242      +/-   ##
==========================================
- Coverage   55.16%   55.08%   -0.08%     
==========================================
  Files          35       35              
  Lines        3350     3340      -10     
==========================================
- Hits         1848     1840       -8     
+ Misses       1233     1232       -1     
+ Partials      269      268       -1     

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