switch to connection strings instead of separate connection parameters

[pashagolub]

Pros

• connection strings are flexible and long time supported
• one can use either key-value or url format
• one can omit any part of connection string, e.g. user, host, etc.
• one can use multi-host strings, e.g. postgresql://host1:123,host2:456/somedb
• one can specify any custom connection parameter, e.g. synchronous_commit
• the host part may be either a host name or an IP address, e.g. postgresql://[2001:db8::1234]/database
• it's easier to store in the back-end and manipulate

Cons

• users should know how to use it (not a valid reason, because pw3 is not a newbie thingy anyway)

TODO

• remove single connection options, e.g. host, post, etc.
• add configuration kind type: url, file, folder
• update config validation
• update GetPostgresDBConnection()
• update main.go
• update patroni.go
• update database schema
• update UI

[pashagolub]

@denys-holub would you please fix the UI part. To keep things simple:

• delete all connection edits
• replace them with only one memo with title "Connection string"
• rename "Password encryption" to "Encryption"
• remove SSL tab completely

We will try to add syntax highlighting and other features later