Trim contents of slack api urls from files

[srhb]

Trailing newlines will almost always be present in a posixish file, so trim it away at first opportunity to prevent having invalid URLs we won't discover until a notification is sent.

[gotjosh]

Can you please fix the DCO and create a test case for this? Thanks!

[srhb]

Sure, here you go :smile_cat:

[simonpasquier]

you'd need to fix the test

notify/slack/slack_test.go:88:12: undeclared name: `ioutil` (typecheck)
	f, err := ioutil.TempFile("", "slack_test_newline")
	          ^
make: *** [Makefile.common:198: common-lint] Error 1

Exited with code exit status 2

[simonpasquier]

thanks!