Implement bot-user support

[mtyurt]

I have added a checkbox which indicates given token belongs to a bot user or not.

But why?

Recently, I have figured out that if I put @username to channel parameter, the plugin sends a direct message via @slackbot. But I didn't want @slackbot to send me a dm, a bot (preferably named jenkins-bot) should send me a dm. So here it is.

• In global config and advanced job config, there is a checkbox named "Is Bot User?" (false by default).
• If this checkbox is checked, the plugin will send a post request to https://slack.com/api/chat.postMessage .
• Also, I added all parameters to request URI if "bot-user" is true, leaving the original case as it is, because sending parameters in JSON body didn't work for chat.postMessage endpoint, I assume there is a special case for "/services/hooks/jenkins-ci".
• Lastly, there are a few string equals improvements that I couldn't resist.

[rborer]

Nice addition, I was wondering how to achieve bot messaging not coming from slackbot, so +1 for me!

[rborer]

You may even be tempted to use StringUtils.isBlank(buildServerUrl) (https://commons.apache.org/proper/commons-lang/javadocs/api-2.6/org/apache/commons/lang/StringUtils.html#isBlank(java.lang.String)) :-)

[kmadel]

Any updates to my comments? Also there are a number of conflicts.

[mtyurt]

@kmadel merge conflicts are now resolved. I'm confused about the comments, am I the target of these comments? Because I did not get any notification about it.

[kmadel]

@mtyurt Not sure why you didn't get notified. But yes, those comments were directed at you. Need 'Is Bot' specific help file. Also, please update the README.md to document this feature. My plan it move all documentation into README and direct users from the Jenkins Wiki to the README. And Thanks!

[mtyurt]

@kmadel from your comment I understand all documentation should be in README file, however there aren't any documentation regarding configurations in README so far. I'll change the screenshot including bot user option, if we want to add bot-user explanation to README, I can add it after Security section as Bot User Support with a brief explanation.

Also, the screenshots in current README file is not up-to-date. Adding new credentials screen has a different order of fields. I'm uploading the screenshot I took while testing with hpi. The credentials has some text prefix, I have no idea why.

[kmadel]

@mtyurt Yes, please add Bot User Support section to README as you mentioned - I know the README is still very much lacking, but we will get it where it needs to be one merge at a time.
Thanks!

Also, would be nice to get a few more thumbs up from a code review perspective.

[mtyurt]

@kmadel it's done, can you guys have a look please?

[samrocketman]

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