Improve import-db/export-db with --target-db and --no-drop flags, fixes #1961, fixes #1652

[rfay]

The Problem/Issue/Bug:

#1961 requested the ability (later the default) of dropping tables
#1652 requested an additional flag to target a secondary database

How this PR Solves The Problem:

• Goes back to import-db dropping all tables as the default
• Adds --no-drop flag for when that's not what's wanted
• Adds --target-db to target another database (and create it if necessary)
• Minor docs added to the FAQ section about working with additional databases
• Adds optional "project" arg to both import-db and export-db
• Adds --target-db also to ddev export-db

Manual Testing Instructions:

ddev export-db <project> >someproject.sql.gz
ddev import-db <someproject.sql.gz
ddev import-db --target-db newdb --src someproject.zql.gz, verify contents of "newdb"
ddev import-db another smaller one. Only the new tables should still exist.
ddev import-db a normal db
ddev import-db --no-drop a smaller one with different tables. The union of the tables should now exist.
ddev import-db --target-db=somedb and verify that "somedb" now exists and has contents of the import.

Automated Testing Overview:

• TestDdevImportDB was expanded to cover this territory
• TestCmdImportDB was expanded to handle import from named project
• TestCmdExportDB was added with basic test of ddev export-db
• TestDdevExportDB expanded

Related Issue Link(s):

#1961
#1652

Release/Deployment notes: