Fix all issues and improvements

The following changes were made:

Make url optional on event creation
Insert a default image on the event creation page
Complete list of african countries in registration forms
add a All button to get back to all CSO after selecting one group in cso-directory page
Article: define/create list of tags and select multiple tags during article creation
Switch between what we do and Our appraoch in about page
Learn button of home page to lead to About us page
Lastest experts and lastest CSO in home page lead to expert details and cso details pages respectively
Newsletter signup implemented
All obligatory items on registration forms have asterisks (*)
All values on registration forms are persisted
English translations on CSO directory page cso domains
Change confirm and pay on arrival button to Book and pay on arrival
Add booking status, with default as pending and other as confirmed
Booking: added message that a confirmation will be sent to the person booking
Add confirmation button for the admin with sending a message to the booking person to confirm
Expert Work duration accepts only numbers
Comment section added to blog posts
Active links affected on directory detail pages
Statistics on about us page: CSO registered, Number of trainings carried out, Numbers of events organized

whilesmart / lacsodec