frontend: Do not re-create services in EXTSVC_CONFIG_FILE

Created by: mrnugget

This fixes #5134 (closed) by using the suggested changes and extending them to also allow the in-place modification of external services in case they have the same display name.

I tested this with 3 GitHub external services and 1 AWS CC external service in external-services-config.json defined and then booted up frontend with debug log messages enabled:

msg="Adding external service" displayName="AWSCODECOMMIT #1"
msg="Adding external service" displayName="GITHUB #1"
msg="Adding external service" displayName="GITHUB #2"
msg="Adding external service" displayName="GITHUB #3"

After updating GITHUB #2 in the JSON and restarting:

msg="Updating external service" id=3 displayName="GITHUB #2"

Without changing the JSON and restarting:

<no changes>

After updating GITHUB #1 in the JSON and restarting:

msg="Updating external service" id=2 displayName="GITHUB #1"

After removing GITHUB #3 from JSON and restarting:

msg="Deleting external service" id=4 displayName="GITHUB #3"

After re-adding GITHUB #3 to JSON and restarting:

msg="Adding external service" displayName="GITHUB #3"