Specify download subdirectory for workspace

[knapsu]

Second pack of fixes and improvements from Housemarque.

• Improvement: Configuration form for both freestyle and pipeline projects allows now to specify a directory into which the repository content will be downloaded. This is a must have if willing to have a project using multiple workspaces.
• Fix: Allowing to explicitly specify the directory fixes several issues that we have encountered when having multiple workspaces and subdirectories created magically from workspace name. This includes workspace name collision, crashes for concurrent build executions, need for custom scripts to check the workspace directory names, etc.
• Fix: Workspace name was not set when additional workspaces were added.
• Improvement: Secure validation. Protect against CSRF attacks. Ensure only users with configuration permissions can check/validate repository access.
• Improvement: Smart form validation for directory name. Field can be empty unless multiple workspaces are being used. Form validation will handle this dynamically as user enters data into the form. Need to add a check if directory name is duplicated by additional workspace.

[knapsu]

Jenkinsfile needs to be corrected in main branch to make continuous integration build run properly for this PR.

[knapsu]

Looks good to me.