Fix 87 initialising transcript media - component interface

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#87

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As explained in the QA section

The component at a minimum needs two params to function properly and to be able to save to local storage. Transcript as transcriptData (+ sttJsonType) and media asmediaUrl.

We'd normally expect both to be provided at the same time, but there might be edge case where the component is initialized with the first one, and then subsequently receives the second one (or the other way around).

This PR addresses this issue, making it possible to initialise the component with either both, or with one or the other first subsequently followed by the other.

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QA testing notes

✅ Both at same time - media+transcript

Steps:

• click 'clear local storage'
• 'Load demo'
• Edit text
• refresh browser
• 'Load demo'

Expected Results:

• Expect the text change in step 3 to have persisted

✅ Media first - local media

Steps:

• click 'clear local storage'
• 'Load Local Media' + 'Chose File'
• 'open Transcript Json' + 'Choose file'
• Edit text
• refresh browser
• repeat step 2 and 3

Expected Results:

• Expect the text change in step 4 to have persisted

✅ Media first - url

Steps:

• click 'clear local storage'
• 'Load media from Url' and add this url: https://download.ted.com/talks/KateDarling_2018S-950k.mp4
• 'open Transcript Json' + 'Choose file'
• Edit text
• refresh browser
• repeat step 2 and 3

Expected Results:

• Expect the text change in step 4 to have persisted

✅ Transcript first - local media

Steps:

• click 'clear local storage'
• 'open Transcript Json' + 'Choose file'
• 'Load Local Media' + 'Chose File'
• Edit text
• refresh browser
• repeat step 2 and 3

Expected Results:

• Expect the text change in step 4 to have persisted

✅ Transcript first - url

Steps:

• click 'clear local storage'
• 'open Transcript Json' + 'Choose file'
• 'Load media from Url'
• add this url: https://download.ted.com/talks/KateDarling_2018S-950k.mp4
• Edit text
• refresh browser
• repeat step 2 and 3

Expected Results:

• Expect the text change in step 4 to have persisted