New features added

[dtamajon]

Issue view:
- Added link at the top of the issue view (EVENT_MENU_ISSUE).

Created view 2:
- Register time for work type and minutes type.
- Added 'info' field in table 'work'
- Substract 'todo' when 'done' is informed
- When minutes_type=='est' => target_date=book_date
- When minutes_type=='done' => done=done+minutes / todo=todo-minutes
- When minutes_type=='completed' (new just for select) => completed_date=book_date / done=done+minutes / todo=0
- Remove own time register (if admin_own_threshold)
- Remove others time register (if admin_threshold)
- Show/Hide all work types on target table (show_all_work_types_on_bug_targets)
- When a 'todo' is defined without 'est', 'todo' is shown in red and with a tooltip containing the error

Config options:
- [bug_view_mode][new] View selection: 1 (original) or 2 (purposed)
- [show_all_work_types_on_bug_targets][new] Show/Hide all work types on target table. Only works on view 2
- [admin_own_threshold][new] Enabled to remove custom registers. Only works on view 2
- [edit_targets_threshold][old] Enable to modify targets over the targets table. It was defined on function config, but now is updatable from config page
- [billable_mandatory_minimun_status] Select the minimun status for billable required shows the error
- [billable_behavior_over_severity] Define behavior of billing requirement according to severity

General:
- More specific errors
- More detailed history actions

Customers:
- Added 'association_mode' field in table 'customer'
- Created 'project_customer' table
- Customer management page updated with 'association_mode' field
- When a new project is created, the customers with association_mode='auto' are attached automatically
- From project, you can attach/detach customers, except those marked with association_mode='all'
- When a issue is resolved or closed, 'all customers' is transformed in the current customers list. It avoids future billing changes when adding a new customer to the project.
- 'paying_customers' is required only if 'is_billable' is set to true, and the issue has adquired the minimum status (currently, the minimum status is defined hardcoded in 'pm_bug_is_billable_affecting_required_paying_customers')

Reports:
- Added 'dashboard' report
- Error control added for 'Resource progress' report
- Added validation in 'print_progressbar_span' and 'print_progress_span' to avoid error when no resources are defined (only exploited when $g_show_detailed_errors=ON)
- Show hourly rate and cost only is user has 'view_billing_threshold' permissions

Report 'Time registration overview':
- Layout adjustment to avoid paddings and margins
- Collapsible panels

Report 'Billing':
- Shows only issues marked as billable

Account configuration:
- Moved the unavailability management to a self account preferences menu
- Added list of future and past unavailability, allowing to remove just for future