Move the JIRA configuration from constants to LocalSettings.php

This modification replaces the constants:
- `JIRAEndpoint`
- `JIRAUserName`
- `JIRAUserPassword`

by global variables that need to be defined inside *LocalSettings.php*:
```php
#JIRA Connector
$jiraURL="https://jira.atlassian.com";
$jiraUsername=NULL;
$jiraPassword=NULL;
```

In addition it adds support for anonymous access to a JIRA instance by assigning a `NULL` value to `$jiraUsername`.