Long-term graph changes

[rdwebdesign]

By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes.
• I am willing to help maintain this change if there are issues with it later.
• I give this submission freely and claim no ownership.
• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)
• I have Signed Off all commits. (git commit --signoff)

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What does this PR aim to accomplish?:

Fix #1968

How does this PR accomplish the above?:

• this PR changes the minimum date allowed for the "daterangepicker", ensuring that the beginning of the displayed graph matches the oldest data contained in the database.

• The maximum date is limited to "today" (the limits apply to all ranges).

• a call to the API retrieves the oldest date from the database (getMinTimestamp);

• this value is used to limit the daterangepicker selector.

• the code that controls the "interval" between ticks was changed too.

What documentation changes (if any) are needed to support this PR?:
none

[yubiuser]

Before:

After:

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-12-web-v5-9-and-core-v5-7-released/51795/1

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/show-oldest-date-instead-of-dec-31st-1969-for-all-time-in-long-term-data/32807/2