Improve Text Post thumbnail generation

[libertysoft3]

These posts failed to find a thumbnail for unknown reasons. Loosen any restraints so that we generally always find something. Image dimension restrictions may be in place, and/or we're only checking the first X bytes of a text post for an image link.

• https://saidit.net/s/Health/comments/3c1j/why_isnt_the_msm_taking_advantage_of_this_flu/
• https://saidit.net/s/whatever/comments/3eby/i_was_social_distancing_before_it_was_cool_tshirts/
• https://saidit.net/s/ClimateSkeptics/comments/3aed/esa_study_reveals_the_2019_horrific_amazon_fire/
• https://saidit.net/s/Solutions/comments/32zn/nike_reveals_space_hippie_sustainable_sneakers/

Thanks /u/JasonCarswell