Addition of a provisioning feature which modifies the account 'Label' on Yealinks

[levishores]

In the course of configuring an extension and device in Fusion PBX, the system automatically populates the Label and Display Name fields in the device from the extension's data. (Notably: Effective CID Name goes to 'Display Name')

In a Yealink, if the Label is populated, the Display Name is not used on the screen - it is apparently only intended for sip calls where the phone would be controlling the outbound CID.

In small offices with shared workstations (of which most of our deployments are), the combination of 'friendly labels' on BLFs is helped greatly if the user can quickly identify the friendly name of a given phone.

Without the feature in this PR, the default behavior after provisioning a Yealink phone looks as so:

(T53W)

(T46)

This PR adds two variables to Default Settings:
yealink_label_content_concat - used to toggle the feature; disabled and set to 0 by default
$yealink_label_content_concat_middle - the character(s) intended to concatenate between the Label and Display name

and modifies several of the yealink provisioning files such that if the above is configured, the Label (numeric extension) and Display Name (Freindly Name) of the account defined in Fusion are concatenated: {label}{concat_middle}{display_name}

This allows for the Fusion UI and extension/device building process to remain untouched and to not impact other phones, and creates the following effect on provisioning:

(T53)

(T46)

Notably, it would be desirable to have the 'middle' value populated by a colon followed by a space, however Fusion [dutifully] seems to drop trailing spaces. I attempted to resolve with the HTML non-breaking-pace string, but that just passed those characters to the phone. The lack of the space isn't a major issue though.

I acknowledge that all of this could be done by simply editing the 'label' in the account definition of the device, however that stands in the way of rapid, easy deployments and increases the chances for user error by increasing touch points.

Also, I'm including a Default Setting for a previously merged PR:

yealink_blf_active_backlight