replaced named level with progression when serializing script dsl

[davidsbailey]

Starts https://codedotorg.atlassian.net/browse/PLAT-621.

Background

What are named levels and progressions?

1. By default, levels appear in a lesson like this:
   

2. If a progression is applied to multiple consecutive levels, it looks like this:

• code-dot-org/dashboard/config/scripts/csp5-2018.script

  Lines 27 to 31 in ab3e15e

  	level 'Intro to Debugging and Common Problems_2018', progression: 'Intro to Debugging and Common Problems'
  	level 'CSPU5_predict_applab_onEventWithWrongID_2018', progression: 'Intro to Debugging and Common Problems'
  	level 'CSPU5_U3L13 - Debugging IDs case sensitive_2018', progression: 'Intro to Debugging and Common Problems'
  	level 'CSPU5_Debugging predict unexpected behavior - two onEvents to one_2018', progression: 'Intro to Debugging and Common Problems'
  	level 'CSPU5_U3L13 - Debugging 3_2018', progression: 'Intro to Debugging and Common Problems'

• 

3. If a progression is applied to exactly one level, it appears on a line by itself along with the progression name:

• code-dot-org/dashboard/config/scripts/csp5-2018.script

  Line 17 in ab3e15e

  level 'Introduction to Design Mode_2018', progression: 'Intro to Design Mode in App Lab'

• 

4. When a level is "named", it appears on a line by itself along with a custom display name, just like a progression containing only one level. however, in this case the display name is determined by the display_name property on the level:

• code-dot-org/dashboard/config/scripts/dance-extras.script

  Line 8 in b5c8251

  level 'Dance_Party_extras_intro', named: true

• code-dot-org/dashboard/config/scripts/dance_party_extras_intro.external

  Line 2 in eafc78a

  display_name 'Keep On Dancing!'

• 

5. If a named level has no display name, the level name is shown instead.

6. If a named level also has a progression, the progression takes precedence:

• 

• code-dot-org/dashboard/config/scripts/csp5-2018.script

  Lines 181 to 183 in ab3e15e

  	lesson 'Building an App: Color Sleuth', display_name: 'Building an App: Color Sleuth', has_lesson_plan: true
  	level 'CSP U5L10 SFLP', named: true
  	level 'Color Sleuth - Planning the App_2018', progression: 'Color Sleuth - Planning the App', named: true

Why do we want to get rid of named levels?

The new script editor does not support named levels. named levels are also redundant functionality, because the same goal can be accomplished by adding a progression. adding a progression is superior because it does not require finding the level and adding a display name to it, rather the name can be added without leaving the lesson editor.

Description

The plan is as follows:

• (this PR) when serializing script DSL, convert named syntax to progression syntax
• run Script.all.each(&:write_script_dsl)on levelbuilder, which will remove named syntax from all .script files
• remove code for parsing named syntax when seeding scripts from script DSL

Testing story

• new unit tests cover the new behavior
• run step 2 locally, then spot-check a few different scripts with different combinations of named and progression settings to confirm there is no visible change to end users

[Hamms]

I'm not super familiar with these properties of levels, but the explanation makes sense and the plan for gradual conversion seems reasonable!