Acknowledge possible custom time interval

[DL6ER]

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What does this PR aim to accomplish?:

We currently show a static text "Queries over last 24 hours". However, a user might use MAXLOGAGE to change the interval of shown data to 48 hours or even more. In this case, the title of the graph should be reflecting this.

How does this PR accomplish the above?:

We try to obtain the correct value from /etc/pihole-FTL.conf. If this is not possible (because it is not set), we default to show "24 hours" in agreement with the default setting FTL uses in this case.

[pralor-bot]

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

https://discourse.pi-hole.net/t/regex-wildcard-blocking-nach-update-auf-4-0/11521/21