[CHORE] Remove start_code useages

[Felienne]

#4913 and #4920 removed start_code from content (and needed to be merged to reopen Weblate) but the code still has a few references:

There are a number of other places where start_code is still referenced. Are you interested in removing these as well, or do you just want the current PR shipped?

tools/rewrite-content-yaml.py
52:    "start_code",

website/frontend_types.py
86:    start_code: str
103:            start_code='',  # Teacher adventures don't seem to have this

content/adventures/adventures.schema.json
43:        "start_code": { "type": "string" },

static/js/app.ts
247:      adventure.start_code = programFromLs.code;
556:          adventure.start_code = code;
1928:    theGlobalEditor.contents = adventure.start_code;
2027:  const programChanged = theGlobalEditor.contents !== adventure.start_code;
2037:  const programSuspiciouslyShrunk = wasSavedBefore && theGlobalEditor.contents.length < adventure.start_code.length * su [...]
2086:    adventure.start_code = code;
2093:    adventure.start_code = code;

static/js/types.ts
38:  start_code: string;

app.py
165:                start_code=adventure['levels'][level].get('start_code', ""))
182:    Mutates the adventures in-place, by setting the 'save_name', 'start_code'
208:        adventure.start_code = program.code
1224:                start_code=loaded_program.code,
1391:                start_code=loaded_program.code,
1515:                                               start_code=code,

Originally posted by @rix0rrr in #4920 (comment)